Add ListFlavors() and acceptance test.

This is required to support server creation acceptance testing.  Without
the ability to detect flavors automatically, it isn't possible to
automate acceptance testing, as then a human operator would be required
to manually intervene while testing.
diff --git a/interfaces.go b/interfaces.go
index ee02baf..649e756 100644
--- a/interfaces.go
+++ b/interfaces.go
@@ -26,5 +26,9 @@
   // Images
 
   ListImages() ([]Image, error)
+
+  // Flavors
+
+  ListFlavors() ([]Flavor, error)
 }