When a Nomad service starts it tries to establish a connection with
servers, but it also runs alloc runners to manage whatever allocations
it needs to run.

The alloc runner will invoke several hooks to perform actions, with some
of them requiring access to the Nomad servers, such as Native Service
Discovery Registration.

If the alloc runner starts before a connection is established the alloc
runner will fail, causing the allocation to be shutdown. This is
particularly problematic for disconnected allocations that are
reconnecting, as they may fail as soon as the client reconnects.

This commit changes the RPC request logic to retry it, using the
existing retry mechanism, if there are no servers available.