title: composer update keeps overwriting parameters.yml when using custom values
body: <p>I have some custom entries in my parameters.yml, every time I run a composer update, it wants to add the missing entries, which even worse overwrites my custom entries.  How can I stop this?</p>
  
  <p>e.g. before composer update </p>
  
  <pre><code>#parameters.yml
  # Env = GLOBAL
  parameters:
  # DB settings - GLOBAL
  database_driver: pdo_mysql
  database_host: 127.0.0.1
  database_port: 3306
  
  #mailer settings
  mailer_to: yo@yo.com
  mailer_from: yo@site.com
  mailer_transport: smtp
  mailer_subject: ":-| Something is broken!"
  
  # Framework use - GLOBAL
  locale: en
  </code></pre>
  
  <p>after</p>
  
  <pre><code>#parameters.yml
  # This file is auto-generated during the composer install
  parameters:
  database_driver: pdo_mysql
  database_host: 127.0.0.1
  database_port: 3306
  mailer_transport: smtp
  locale: en
  database_name: 
  database_user: 
  database_password: 
  mailer_host: 
  mailer_user: 
  mailer_password: 
  secret: 
  </code></pre>
  
  <p>Luckily my repo and my working file through my ide are two different files and need to be synced.  after i run composer update, when i run a sync, i download all files to my IDE except for the parameters.yml, that one i push back up to overwrite the one that was just auto created.  </p>
  
  <p>I would like to eliminate this hassle, as I keep my db passwords on a per/env file.</p>
  
  <p><strong>EDIT:</strong> When I try to fill in the unneeded vars as dummy values by nulling them out, it backfires as well. </p>
  
  <pre><code>$ composer update
  Loading composer repositories with package information
  Updating dependencies (including require-dev)
  Generating autoload files
  Updating the "app/config/parameters.yml" file
  Some parameters are missing. Please provide them.
  database_name: null
  database_user: null
  database_password: null
  mailer_host: null
  mailer_user: null
  mailer_password: null
  secret: null
  
    [Symfony\Component\DependencyInjection\Exception\ParameterNotFoundException]
    You have requested a non-existent parameter "mailer_from".
  
  Script Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::clearCache handling the post-update-cmd event terminated with an exception
  
    [RuntimeException]
    An error occurred when executing the "'cache:clear --no-warmup'" command.
  </code></pre>
  
  <p>The custom param is causing problems itself as well. </p>
  
  <p>So now I go to peek at my newly generated file, and it looks like this</p>
  
  <pre><code># This file is auto-generated during the composer install
  parameters:
      database_driver: pdo_mysql
      database_host: 127.0.0.1
      database_port: 3306
      mailer_transport: smtp
      locale: en
      database_name: null
      database_user: null
      database_password: null
      mailer_host: null
      mailer_user: null
      mailer_password: null
      secret: null
  </code></pre>
  
  <p>But what i need it to say after regenerating ideally is this</p>
  
  <pre><code>/**
  * my custom comment and copyright/legal tamper warning
  */
  #  Env = GLOBAL
  parameters:
    # DB settings - GLOBAL
    database_driver: pdo_mysql
    database_host: 127.0.0.1
    database_port: 3306
  
    #mailer settings
    mail_to: me@site.com
    mail_from: site@site.com
    mail_transport: smtp
    mail_subject: ":-| Something is broken!"
  
    # Framework use - GLOBAL
    locale: en
  </code></pre>
  
  <p>The rest of my parameters are located on another parameters file, that is dynamically included depending on what env loads, and has different db creds on them each.</p>
  
  <p><strong>EDIT</strong> <em>per @AlpineCoder answer:</em> 
  my latests findings why adding dummy values doesnt work</p>
  
  <p>When I add this to my parameters.yml</p>
  
  <pre><code>#dummy values  to avoid regeneration
      database_name: dummyvalue
      database_user: dummyvalue
      database_password: dummyvalue
      mailer_host: dummyvalue
      mailer_user: dummyvalue
      mailer_password: dummyvalue
      secret: dummyvalue
  </code></pre>
  
  <p>It no longer asks me to fill in these values, thats great, however, it still regenerates the file, deleting all my custom values, leaving me with this</p>
  
  <pre><code># This file is auto-generated during the composer install
  parameters:
      database_driver: pdo_mysql
      database_host: 127.0.0.1
      database_port: 3306
      mailer_transport: smtp
      locale: en
      database_name: dummyvalue
      database_user: dummyvalue
      database_password: dummyvalue
      mailer_host: dummyvalue
      mailer_user: dummyvalue
      mailer_password: dummyvalue
      secret: dummyvalue
  </code></pre>