PCBC-208: Add flushBucket() to ClusterManager

Change-Id: I772feb14f0a5249f42058486b401ae56b2704c53
Reviewed-on: http://review.couchbase.org/25059
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>