Add delete method to Flavor class

In tempest, some tests use delete() method of each resource class
(Server, SecurityGroup, etc.) for cleaning up when each test finishes.
Now we are adding some tests, which create a Flavor instance, to tempest
and we need the delete() method of Flavor class.

Fixes bug #1218156

Change-Id: I210d62aa45510858346315046cf57ea7b1de7b7b
Merge "Add delete method to Flavor class"