Use skip_checks for live-migration feature in test
Use skip_checks for live-migration feature in test

flag compute_feature_enabled.live_migration is class-wide,
so it make sense to use skip_checks method for class, instead
of decorating each test with skipIf, skipUnless, etc.

Change-Id: I62006b7e636b1773f4f41709cfec1fd6daae3c4f

cisco-openstack/tempest,masayukig/tempest,Juniper/tempest,sebrandon1/tempest,vedujoshi/tempest,Tesora/tesora-tempest,cisco-openstack/tempest,bigswitch/tempest,sebrandon1/tempest,LIS/lis-tempest,openstack/tempest,zsoltdudas/lis-tempest,vedujoshi/tempest,zsoltdudas/lis-tempest,openstack/tempest,bigswitch/tempest,masayukig/tempest,Juniper/tempest,LIS/lis-tempest,Tesora/tesora-tempest