Enable a uuid flavor

Current tempest configuration enforces a flavor id should be integer,
but we can create a uuid flavor. For example, the following command
creates a flavor with auto-generated uuid:

  $ nova flavor-create m3.small auto 512 0 1

This patch enables a uuid flavor by changing a flavor_id type from
integer to string.

Change-Id: I6567ef3086710508ef4215e93634601119ce3402
diff --git a/tempest/config.py b/tempest/config.py
index 6af7d51..823550d 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -117,11 +117,11 @@
     cfg.StrOpt('image_ref_alt',
                default="{$IMAGE_ID_ALT}",
                help="Valid secondary image reference to be used in tests."),
-    cfg.IntOpt('flavor_ref',
-               default=1,
+    cfg.StrOpt('flavor_ref',
+               default="1",
                help="Valid primary flavor to use in tests."),
-    cfg.IntOpt('flavor_ref_alt',
-               default=2,
+    cfg.StrOpt('flavor_ref_alt',
+               default="2",
                help='Valid secondary flavor to be used in tests.'),
     cfg.StrOpt('image_ssh_user',
                default="root",