CBQE-1274: support for teardown in systest

tear down ddocs/buckets/xdcr
for example..

"0" :
{
    "name" : "local_teardown",
    "desc" :  "local_teardown",
    "teardown" : {"ddocs" : ["default/ddoc1",
                   "saslbucket/ddoc2"],
                   "buckets" : ["default",
                   "saslbucket"],
                   "xdcr_dest_clusters" : ["remote"]}
}

Change-Id: I7f524f47faeaa1d72899b3ef53c8a1e50311d7bd
Reviewed-on: http://review.couchbase.org/26160
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>