confmon: change nodes_updated to config_updated

This makes the semantics clearer and distinguishes between
configure_nodes() and config_updated() (which was previously
nodes_updated())

Change-Id: I407a521f95c013e43aebe1c84c08f447a0d49b67
Reviewed-on: http://review.couchbase.org/37591
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>