MB-24648: Fail node add when its already part of cluster.
MB-24648: Fail node add when its already part of cluster.

Currently if a node is present in the nodes_wanted list and for some
reason its config gets deleted we allow the node to be re-added to
the cluster without doing a failover first. This causes data loss.
With this changeset we check first if the node is present in
nodes_wanted and add only if it doesn't already exist.

Change-Id: Id3367e1a0865db245e695d0e5134904432cc642a
Reviewed-on: http://review.couchbase.org/79972
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Tested-by: Amit Krishnan <2d9be4f8a4cc2611c6b78293f950d6eb7edc7196@gmail.com>
Reviewed-by: Poonam D <6855f09986fe28ed7e8cdd9d60db878d898703d5@couchbase.com>

membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server