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/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 251336e..e1edd01 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -236,10 +236,10 @@
 # value)
 #image_ref_alt={$IMAGE_ID_ALT}
 
-# Valid primary flavor to use in tests. (integer value)
+# Valid primary flavor to use in tests. (string value)
 #flavor_ref=1
 
-# Valid secondary flavor to be used in tests. (integer value)
+# Valid secondary flavor to be used in tests. (string value)
 #flavor_ref_alt=2
 
 # User name used to authenticate to an instance. (string