Remove some mentions of Gear.
Rename GearEnvironment, remove vestigial links references and get rid of
more gear references.
Remove more references to links.
Get rid of Unit.sub_state, making Unit.activation_state documentation
more accurate.
Docker client currently only does active/inactivate states, so don't
bother with anything else.
Get rid of remaining gear references.
Lint fix.
Address review comments: Various docstring fixes.
Merge remote-tracking branch 'refs/remotes/origin/master' into final-geard-purge-608

Conflicts:
	flocker/node/test/test_deploy.py
Address review comment: link to follow up issue.
Merge pull request #811 from ClusterHQ/final-geard-purge-608

Purge almost all of remaining geard idioms and mentions.

Fixes #608.