Discovery: only retry join for remote exceptions

When a node tries to join a master, the master may not yet be ready to accept the join request. In such cases we retry sending the join request up to 3 times before going back to ping. To detect this the current logic uses ExceptionsHelper.unwrapCause(t) to unwrap the incoming RemoteTransportException and inspect it's source, looking for `ElasticsearchIllegalStateException`. However, a local `ElasticsearchIllegalStateException` can also be thrown when the join process should be cancelled (i.e., node shut down). In this case we shouldn't retry.

See http://build-us-00.elasticsearch.org/job/es_g1gc_master_metal/499/testReport/junit/org.elasticsearch.discovery.zen/ZenDiscoveryTests/testNodeFailuresAreProcessedOnce/ for a test that gets confused by the extra join