Change bionic test from dev to gate for 18.05.

Change-Id: Ib4cf1f94b3ba86380f79b5254e21de46f23d3c98

Gnocchi was providing its URL to client charms before its VIP was
completely setup. This change checks that an hacluster relation
exists and if so waits to provide its URL until the hacluster setup
is complete.

Depends-On: I23eb5e70537a62d5b9e5e24d09f37519b63a1717
Change-Id: I3a6991ecb4eca8659c08d5c5d00d35b8d22bf79e
Closes-Bug: #1749280

Minor refactor to drop use of local Memcache relation adapter
now that its been incorporated into charms.openstack.

Change-Id: I2b5d8ed26513574b34c1cdfd8edbe53bb1498343

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

Ensure that the pool name requested matches the service name
for the application, inline with other OpenStack charms.

Change-Id: Ia3ee274b52bdb2cb90a17e43ec5a379e7765efe0