Fix wrong zuul_project name in mistral gate script
Fix wrong zuul_project name in mistral gate script

$ZUUL_PROJECT should be openstack/mistral

Change-Id: Ice2440ee4cacfa09d662403a20944ac4d0df9e0e
Closes-Bug: #1468572

dennybaa/mistral,StackStorm/mistral,StackStorm/mistral,openstack/mistral,dennybaa/mistral,openstack/mistral