text: Title: [RFC] Method or property listing all defined environments
  Body: **Is your feature request related to a problem? Please describe.**
  
  I'm trying to build a argparse argument that has a list of the available environments as choices to the argument. But I don't see any way to get this at the moment.
  
  **Describe the solution you'd like**
  I am proposing 2 features closely related to help with environment choices as a list and to validate that the environment was defined (not just that it is used with defaults or globals).
  
  The first would be a way to get a list of defined environments minus `default` and global. This would make it easy to add to argparse as an argument to choices. I imagine a method or property such as `settings.available_environments` or `settings.defined_environments`.
  
  The second feature would be a method to check if the environment is defined in settings. This could be used for checks in cases you don't use argparse or want to avoid selecting a non-existent environment. Maybe `settings.is_defined_environment('qa')` or similar.
  
  **Describe alternatives you've considered**
  I'm currently parsing my settings file keys outside of Dynaconf and discarding `default` and `global`. But this feels hacky.
  
  **Additional context**
  Since the environment is lazy loaded I wonder if this would be considered too expensive to do at load time. Maybe it makes sense as a utility outside of the `settings` object? Maybe there is a good way to do this without the feature? Maybe I shouldn't be doing this at all? :thinking: 
  