Enable "Start Instance" and "Shut Off Instance" buttons

Now that the shutdown and start actions in the nova-api are sane
we'd like to add support for these actions to Horizon.

Fixes Bug: 1190402
Change-Id: I1dffff3850e3fd45e766836600f6c04ebb2f3944
Merge "Enable "Start Instance" and "Shut Off Instance" buttons"