make janitor cleanup interval configurable
make janitor cleanup interval configurable

can be useful to slow down janitor while dealing with rebalance issues

Change-Id: Ied8916a95e38417729876f23ff0bbc09482bd6f1
Reviewed-on: http://review.couchbase.org/51401
Tested-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>
Reviewed-by: Poonam D <6855f09986fe28ed7e8cdd9d60db878d898703d5@couchbase.com>

membase/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,mschoch/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,membase/ns_server,mschoch/ns_server,mschoch/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,ceejatec/ns_server,nimishzynga/ns_server,ceejatec/ns_server,membase/ns_server,mschoch/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,nimishzynga/ns_server