Live migration support

Currently Horizon only provides support for 'nova migrate', which
involves a reboot, and is also confusing for users who do not know there
are 2 ways of migrating an instance. We should provide another option in
order to support live migration, as enabled by `nova live-migration`.

Implements blueprint live-migration-support

Change-Id: Id83ba9e19461a7217f521a09bde3109428ecbf11
Merge "Live migration support"