Live migrate each instance from one host to other hosts

Using the existing server live_migrate api, this new feature,
adds the ability for admins to live-migrate all running instances
from one host to other hosts. The patch implements the feature
for Nova API V2 and API V3.

Co-Authored-By: Cédric Soulas <cedric.soulas@cloudwatt.com>

Change-Id: Ie8dd1b66fb8eaefa6ff38752b1e4f46bab145820
Implements: blueprint host-servers-live-migrate
Merge "Live migrate each instance from one host to other hosts"