add optional "environment" key to application YAML config
a dictionary of environment variable mappings to be made
available to the application's container
lint fix
explicitly cast environment variable values to strings
lint fixes
addresses various review comments
slight readability improvement
	modified:   flocker/node/test/test_model.py
raise error for non-stringtypes environment variable values
Merge pull request #580 from ClusterHQ/env-vars-application-yaml-569

Add environment variables YAML config for Applications