Correct skip condition for migrate_server
Correct skip condition for migrate_server

CONF.compute_feature_enabled.cold_migration should be checked
for migrate_server, this is to use the right skip condition
for migrate_server.

Change-Id: Iab825968861a867c788930f3ce27c62cd2323d3c

vedujoshi/tempest,Juniper/tempest,vedujoshi/tempest,cisco-openstack/tempest,masayukig/tempest,cisco-openstack/tempest,masayukig/tempest,Juniper/tempest,openstack/tempest,openstack/tempest