Most reactive charms react on identity-service.available but the
current interface distinguishes between available and available.auth. It
is somewhat assumed by most charms that identity-service.available is
equivalent to identity-service.available.auth, as what charms are
concerned with is the ability to authenticate against the cloud.
Collapse the difference between identity-service.available and
identity-service.available.auth.
Continue to set identity-service.available.auth for any charms that may
rely on it.
Change-Id: I494feea5f3ef8706140ce712b1e025e52b0dfbd1
Closes-Bug: #1818113
a48a1e826a