Support access to instances via optionally enabled serial console
feature provided in Nova.
Seria console access is enabled using a new config flag; this flag
plus the required base_url for the nova-serialproxy are also passed
over the cloud-compute relation for use in nova-compute units.
This is only supported in OpenStack Juno or later, and replaces
the standard output to the nova console-log.
Change-Id: I3bfcca88bd6147be337e6d770db7348170b914e6