MB-100: track 'Broken pipe' in VBucketAwareMemcached

we have to track error: [Errno 32] Broken pipe at the end of the rebalance out and reset_vbuckets if it's so

http://qa.hq.northscale.net/job/centos-64-2.0-new-rebalance-mixed-cluster/50/consoleFull

Change-Id: Ibf5b889b9ac9007ad2df1d94d9a5d2909025fa18
Reviewed-on: http://review.couchbase.org/24960
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>