expose graceful failover as rebalance subtype in tasks

Change-Id: Icfaa5c7856242e38ade189fb02e39bc6f84640e1
Reviewed-on: http://review.couchbase.org/37266
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>