Implement feature flags for Juju

Proposal for a feature flag implementation in Juju.

Flags are specified in the environment.  Once an environment is bootstrapped with a particular feature flag set, it cannot be changed.  All agents started in that environment will  have the same feature flag set.

NOTE: windows isn't done yet as I haven't figured out how.

(Review request: http://reviews.vapour.ws/r/531/)