provide stable mechanism to retrieve admin credentials

We were relying on a method like this to retrieve the keystone password:

keystone_machine=$(juju status keystone/0 | awk '$1 == "public-address:" {print $2; exit}')
export OS_PASSWORD=$(juju ssh keystone/0 "sudo cat /var/lib/keystone/keystone.passwd 2>/dev/null" | tr -d '[:space:]')

It looks like the behaviour of the keystone charm changed - /var/lib/keystone/keystone.passwd isn't being populated anymore.

We are running keystone in non-HA mode.

Charm source: lp:charms/trusty/keystone
Version: revno 123

The charm config explicitly suggests against setting the password via config except for testing. Also, the config setting is only respected if it's present before install autogenerates credentials.

We need a stable mechanism to extract keystone credentials (aka admin .novarc) to the controlling juju environment.