Enable instance serial console based on flags provided from
nova-cloud-controller charm.
This is only supported in OpenStack Juno or later, and replaces
the standard output to the nova console-log.
Change-Id: I147166d9b273964d34ba81724b0311f269b9fea0
Depends-On: I3bfcca88bd6147be337e6d770db7348170b914e6