Install OpenStack using snaps. By setting openstack-origin to
snap:track/channel or snap:track the charm will use snaps to
install rather than debs. If channel is left off it defaults to
stable. For example: snap:ocata/edge will install the edge version of
Ocata and snap:pike will install the stable version of Pike.

Charm helpers sync for snap related helpers.

Change-Id: I6e3540e4ffe081540404f91061e5c9b7039b3eac