Add environment variables YAML config for Applications

fixes #569 
add optional "environment" key to application YAML config
a dictionary of environment variable mappings to be made
available to the application's container
I ran flake8 on this file and didn't get any complaints. Perhaps you (or I) have an old version. Please check and revert these changes if possible.
```
[richard@drax flocker]$ flake8 flocker/node/_config.py
[richard@drax flocker]$ flake8 --version
2.0 (pep8: 1.5.6, mccabe: 0.2.1, pyflakes: 0.7.3)
```