Subordinates reboot

[Benjamin Saller]

After many discussion a series of improvements are proposed to the
current prototype of the subordinate services feature set.

lp:~bcsaller/juju/subordinate-spec

Includes 3 new documents which would benefit from review and comments.

docs/source/drafts/subordinate-services.rst

  Explains the high level overview and usage of the new features

docs/source/drafts/subordinate-internals.rst

  Contains a more detailed list of changes and additions to the system

docs/source/drafts/implicit-interfaces.rst

   Outlines the small feature dependency on implicit interfaces used
in the feature set.

https://codereview.appspot.com/5541054 has the full review

Please take a look,
Ben