Promote devstack centos7 to non-voting check job

Change Ibff757c62b64dfe18d66dd7b15fd7cf48ecc502d will provide
redundancy between the hp & rax clouds for centos7 nodes.  Promote
this job to a non-voting check job for devstack and devstack-gate
changes.

Change-Id: I71e08fb511f7f524c7d72a9f308dbe3890b620a4
Merge "Promote devstack centos7 to non-voting check job"