Skip migrate server invalid state if resize is off

The migrate (aka cold migration) can be considered a special case of the
resize with a flavor_id equals to None. For that reason to perform
the migrate tests we need to be sure that the resize is enabled.
This patch adds a skip decorators to verify that.

Change-Id: I3a02b1bd210e3934dfd61d4182b301cc33d8a962
diff --git a/tempest/api/compute/admin/test_servers_negative.py b/tempest/api/compute/admin/test_servers_negative.py
index cccaf13..f4d010e 100644
--- a/tempest/api/compute/admin/test_servers_negative.py
+++ b/tempest/api/compute/admin/test_servers_negative.py
@@ -127,6 +127,8 @@
                           self.client.migrate_server,
                           str(uuid.uuid4()))
 
+    @testtools.skipUnless(CONF.compute_feature_enabled.resize,
+                          'Resize not available.')
     @testtools.skipUnless(CONF.compute_feature_enabled.suspend,
                           'Suspend is not available.')
     @test.attr(type=['negative', 'gate'])