Make the dogpile caching optional.

If its omitted from the config, then its not used.  This would be useful for
production right now, to turn off caching since its breaking memcached and
other apps.
Simplify dogpile cache interfaces.

There used to be two unique ones.  One for all the connectors, and a second one
just for the bugzilla connector.  This was a holdover from when I was trying to
optimize bugzilla queries by caching them bit-by-bit as they came in.  This
isn't necessary anymore since the ssl timeout error has been fixed.
Merge pull request #1 from fedora-infra/feature/optional-dogpile

Make the dogpile caching optional.