Fall back to Cloud Auto Join when there are no healthy servers

[dadgar]

Feature Description

If a Consul agent detects that there are no healthy servers or it has failed to RPC to any of the servers in its list, it should search for new servers via the cloud auto-join/retry join mechanism.

Use Case(s)

This can solve the scenario in which the set of servers is restarted with new IPs and thus is not a part of the Serf LAN group.

[zlepper]

Another case for this: Azure scale sets for some reason stats a lot of servers, before it scales down. If Consul manages to start before the servers are removed again, it might have references to dead servers, which then prevents anything from working. The only workaround i have found is connecting the servers, removing the data directory, and restarting consul, which is not really a scaleable solution in the long term.