Add zip to Centos buildslave; fix sshd

Change-Id: Ie41f6ea53294337505bf53aefed1b24f04891f10
Reviewed-on: http://review.couchbase.org/44459
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>