Use dogpile.cache to cache the listTaggedRPMS calls if possible

If the same tag is queried with the same event, Pungi can cache the
response and call the API again. Particularly for small composes this
can save up significant amount of time.

Merges: https://pagure.io/pungi/pull-request/1022
Signed-off-by: Jan Kaluza <jkaluza@redhat.com>

setup.py

@@ -62,6 +62,7 @@ setup(
         "lxml",
         "productmd",
         "six",
+        'dogpile.cache',
         ],
     tests_require = [
         "mock",