Remove skip condition when resize is enable

This commit removes the skip condition with flavor_ref and
flavor_ref_alt when the resize feature is enable. Because when
resize feature is enable, the flavor_ref and flavor_ref_alt should be
set properly, otherwise, it is a misconfiguration. So we should fail
but not skip at the situation.

And This commit also add a help message for the resize config to avoid
users confusing.

Change-Id: I8eab2e57eb77e7a1a8dae7ffd63ec95ae6a026c3
diff --git a/tempest/config.py b/tempest/config.py
index 00c69b0..ee4f844 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -353,7 +353,10 @@
                      "serial console output?"),
     cfg.BoolOpt('resize',
                 default=False,
-                help="Does the test environment support resizing?"),
+                help="Does the test environment support resizing? When you "
+                     "enable this feature, 'flavor_ref_alt' should be set and "
+                     "it should refer to a larger flavor than 'flavor_ref' "
+                     "one."),
     cfg.BoolOpt('pause',
                 default=True,
                 help="Does the test environment support pausing?"),