text: Title: [Migrated] Set PYTHON_EGG_CACHE for flask apps during init
  Body: Originally from: https://github.com/Miserlou/Zappa/issues/1412 by [L226](https://github.com/L226)
  
  <!--- Provide a general summary of the issue in the Title above -->
  ## Context
  <!--- Provide a more detailed introduction to the issue itself, and why you consider it to be a bug -->
  <!--- Also, please make sure that you are running Zappa _from a virtual environment_ and are using Python 2.7/3.6 -->
  I discovered that in my Flask deployment, the app deploys fine with e.g. `zappa init; zappa deploy dev` however upon hitting the generated endpoint a failure is returned.
  
  ## Expected Behavior
  <!--- Tell us what should happen -->
  You should be able to get your expected response from whatever endpoint is hit.
  
  ## Actual Behavior
  <!--- Tell us what happens instead -->
  You get this response:
  ```
  "{u'message': u'An uncaught exception happened while servicing this request. You can investigate this with the `zappa tail` command.', u'traceback': ['Traceback (most recent call last):\\n', '  File \"/var/task/handler.py\", line 452, in handler\\n    response = Response.from_app(self.wsgi_app, environ)\\n', '  File \"/tmp/pip-build-LktYrc/Werkzeug/werkzeug/wrappers.py\", line 903, in from_app\\n', '  File \"/tmp/pip-build-LktYrc/Werkzeug/werkzeug/wrappers.py\", line 57, in _run_wsgi_app\\n', '  File \"/tmp/pip-build-LktYrc/Werkzeug/werkzeug/test.py\", line 884, in run_wsgi_app\\n', \"TypeError: 'NoneType' object is not callable\\n\"]}"
  ```
  `zappa tail dev` yields the following:
  
  ```
  [1519342540529] Can't extract file(s) to egg cache
  
  The following error occurred while trying to extract file(s) to the Python egg
  cache:
  
    [Errno 30] Read-only file system: '/home/sbx_user1060'
  
  The Python egg cache directory is currently set to:
  
    /home/sbx_user1060/.python-eggs
  
  Perhaps your account does not have write access to this directory?  You can
  change the cache directory by setting the PYTHON_EGG_CACHE environment
  variable to point to an accessible directory.
  ```
  
  ## Possible Fix
  <!--- Not obligatory, but suggest a fix or reason for the bug -->
  Seems that PYTHON_EGG_CACHE needs to be set as an environment variable to '/tmp'. I solved by including the following in my zappa_settings.json:
  
  ```json
  "environment_variables": {
              "PYTHON_EGG_CACHE": "/tmp"
          }
  ```
  Unsure if this is Flask specific, or if I stuffed up somewhere, or if this is actually expected behaviour...
  
  ## Steps to Reproduce
  <!--- Provide a link to a live example, or an unambiguous set of steps to -->
  <!--- reproduce this bug include code to reproduce, if relevant -->
  1. Make a flask app
  2. `zappa init`
  3. `zappa deploy dev`
  4. poke API endpoint
  
  ## Your Environment
  <!--- Include as many relevant details about the environment you experienced the bug in -->
  * Zappa version used: 0.45.1
  * Operating System and Python version: 4.13.0-32-generic #35~16.04.1-Ubuntu | Python 2.7.12 
  * The output of `pip freeze`:
  * Link to your project (optional):
  * Your `zappa_settings.py`: 
  ```json
  {
      "dev": {
          "app_function": "*****.api.API", 
          "aws_region": "ap-southeast-2", 
          "profile_name": "*****", 
          "project_name": "api", 
          "runtime": "python2.7", 
          "s3_bucket": "*****",
          "environment_variables": {
              "*****": "*****",
              "PYTHON_EGG_CACHE": "/tmp"
          },
          "domain": "*****.*****",
          "cors": true,
          "certificate_arn": "arn:aws:acm:us-east-1:*******"
      }
  ```