provider/azure: use labels to track state servers

StateServerInstances is updated to call AllInstances and then inspect the
cloud service labels. In 1.19 we started recording which cloud services are
state- servers. For backwards compatibility, we load the provider-state
file if we find any old-style instance IDs.

Requires https://github.com/juju/juju/pull/336

(Please only review commit c20271f in this PR.)