'recheck' by whitelisted users now runs tests

A hidden feature is that one can fill a 'recheck' comment in Gerrit
which is would reenqueue the change in the check pipeline.  But that
pipeline does not run tests!

I upgraded Zuul today, we can now combine filters to craft much better
approvals.  This patch move 'recheck' to the test pipeline, as a side
effect only whitelisted folks can use it now.

Zuul doc: http://ci.openstack.org/zuul/zuul.html#pipelines (look for
'require-approval').

Bug: 64015
Change-Id: Ia1158861d564c6ea15ffbc3439f8aa4f345963ce