Sketch of adding environment variables for controlling links.
Implementation.
Merge branch 'config-internal-links-201' into links-environment-variables-91.
Add links to Application in test_deploy.
Add hostname to StartApplication in tests.
lint
_link_environment should be a free function.
Document this.
Merge remote-tracking branch 'origin' into links-environment-variables-91

Conflicts:
	flocker/node/_config.py
Fix gear environment.
Test for fixed gear env.
An attempt at a functional test for the desired ``IFilesystem.reader`` behavior.
Use the service to create the volume.  This should be more realistic.
Add a functional test for the new `Filesystem.snapshots` method (necessary for the other new functional test, as well as for other reasons).
Must construct Snapshots using keyword arguments.
Filesystem.snapshots returns a Deferred.
lint
Add a functional test for links too.
Execute the snapshot command correctly - with an identifier to assign to the new snapshot.
Outline of elk from postgres example