Documentation for applications with environment variables

[wallrj]

Sub-ticket of #95

Add documentation demonstrating how to use the new environment variable support.

• Maybe this is covered by Adds MySQL volume example to docs #550

Add an entry to the "What's new" document.

• http://docs.clusterhq.com/en/latest/advanced/whatsnew.html

Update the configuration document:

• http://docs.clusterhq.com/en/latest/advanced/configuration.html#application-configuration

[dwgebler]

Estimate: Once Buildbot is fixed and environment variables changes merged ( #596 ), estimate for this is approx. 4 hours, including testing with the official Docker MySQL image, updating docs as linked above and updating docs for the (as of yet unmerged) MySQL example tutorial.