Define "supported_instances" for fake compute

Allow fake computes to deploy images with hypervisor_type
defined and set to "fake" and by extension to be used in
multi hypervisor_type deployments.
Mixing fake and "real" computes on the same OpenStack
deployment allows to create many fake instances and some
working instances for testing applications on top of
OpenStack (ie: fake instances for stress tests, working
ones for functional tests).

Closes-bug: #1260771
Change-Id: Idc337c4a7ed024f236ca2b60d91e2c30f7d54536
Merge "Define "supported_instances" for fake compute"