An arbitarary repository can currently be specified, but it was not yet
made clear in the documentation that a corresponding public key for
accessing this repository could be added. This change specifies that
under the description for the openstack-origin option. Public key can
be added by appending to the deb url, so the below example would work:
juju set openstack-origin nova-compute openstack-origin="deb http://ppa
.launchpad.net/billy-olsen/testfix-kilo/ubuntu vivid main|FA0FD8E1"

Change-Id: I262a2164d4f7b37b4185bdee650371de7be50a55
Closes-Bug: 1503440