Assign CentOS 7 DevStack jobs to correct nodes

* modules/openstack_project/files/zuul/openstack_functions.py: The
temporary parameter function for transitioning dsvm jobs from Ubuntu
Precise to Trusty no longer fail to take jobs which specify centos7
into account.

Change-Id: Ic9813fa7abd9e045e16a9838b3d4873941e95795