Evacuate each instance from one host to another

Added a new extension that adds the ability for admins to evacuate an
entire host to another host. This internally uses the
existing server.evacuate api.
The target host is optional so that a free host will be chosen by the
scheduler in the api.
Implements: blueprint evacuate-host

Change-Id: I2352836d01952e281e15edb9bdd1b912106516d6
Delete a quota through admin api.

Exposes the quota-delete api implemented as part of
blueprint admin-api-for-delete-quota
Change-Id: Iab358f0fcf2dfb41bcd9a3a5b73d590d3f55cd6c
Set/Delete metadata on all instances of a host.

Adding a wrapper for all instances of a host, to be able to set/delete
metadata for all instances at once.

Implements blueprint tag-instances-of-host
Change-Id: I3d1a9ab54aad60bfccd0ece2285d145031fb5e15
Merge "Evacuate each instance from one host to another"
Merge "Set/Delete metadata on all instances of a host."
Merge "Delete a quota through admin api."