[cloud-gce] warn message is displayed when not using gce discovery

[dadoonet]

Similar to #13581 (in master branch - 3.0):

GCE plugin now tries to start immediately gce discovery even if we don't set discovery.type: gce.
Not really an issue because I guess that if someone installs the cloud-gce plugin, it's probably to have discovery set to gce.

But we could may be do something better here...

[2015-09-16 13:04:11,048][WARN ][org.elasticsearch.discovery.gce] [Kubik] cloud.gce.project_id is not set.
[2015-09-16 13:04:11,048][WARN ][org.elasticsearch.discovery.gce] [Kubik] cloud.gce.zone is not set.
[2015-09-16 13:04:12,230][WARN ][org.elasticsearch.discovery.gce] [Kubik] cloud.gce.project_id is not set.
[2015-09-16 13:04:12,231][WARN ][org.elasticsearch.discovery.gce] [Kubik] cloud.gce.zone is not set.
[2015-09-16 13:04:12,231][WARN ][org.elasticsearch.discovery.gce] [Kubik] cloud.gce.project_id is not set.
[2015-09-16 13:04:12,231][WARN ][org.elasticsearch.discovery.gce] [Kubik] cloud.gce.zone is not set.
[2015-09-16 13:04:12,234][INFO ][org.elasticsearch.node   ] [Kubik] initialized
[2015-09-16 13:04:12,243][INFO ][org.elasticsearch.node   ] [Kubik] starting ...
[2015-09-16 13:04:12,437][INFO ][org.elasticsearch.transport.netty] [Kubik] Bound profile [default] to address {127.0.0.1:9300}
[2015-09-16 13:04:12,438][INFO ][org.elasticsearch.transport] [Kubik] bound_address {127.0.0.1:9300}, publish_address {127.0.0.1:9300}
[2015-09-16 13:04:12,445][INFO ][org.elasticsearch.discovery] [Kubik] elasticsearch/R_PjKk5fTB2zCV-jPcyQ3w
[2015-09-16 13:04:12,449][WARN ][org.elasticsearch.cloud.gce] [Kubik] disabling GCE discovery. Can not get list of nodes
[2015-09-16 13:04:13,955][WARN ][org.elasticsearch.cloud.gce] [Kubik] disabling GCE discovery. Can not get list of nodes
[2015-09-16 13:04:15,466][WARN ][org.elasticsearch.cloud.gce] [Kubik] disabling GCE discovery. Can not get list of nodes

[xuzha]

Added 2.0, 2.1 labels, see the conversations in #13809