MB-100: prevent getting builds from sustain page

If the sustain pages are not aviable, test automations break.

Change-Id: I921569be5121ef99060723e900f41d3acba051b4
Reviewed-on: http://review.couchbase.org/28796
Reviewed-by: Thuan Nguyen <soccon@gmail.com>
Tested-by: Thuan Nguyen <soccon@gmail.com>