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.
Merge pull request #337 from axw/provider-azure-stateserverinstances-tags

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.