Enable snap installation support for Gnocchi.

The Gnocchi snap provides nginx and uwsgi instead of
Apache2 + mod_wsgi so the config/restart map looks a
little different.

Snap support has been implemented using a subclass of
the GnocchiCharmBase class, which provides common
methods and properties to both deb and snap based
installations.

Snaps are only published for Ocata or later releases
of OpenStack.

Change-Id: I464025a2b72aba8c31a4a97ade39d2b2980c3a92