Use block devices for live migration
The default values conform with OpenStack and Functest gates [1].
It could be set to false if shared storage-based live migration is
supported [2].
It also currently disables cold migration.
[1] http://logs.openstack.org/84/632984/24/check/tempest-multinode-full-py3/
4216037/controller/logs/tempest_conf.txt.gz
[2] https://docs.openstack.org/nova/pike/admin/configuring-migrations.html
Change-Id: I0faa415eba610c20ba54353ac7ba7b0e83722c32
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>