Cleanup some devstack experimental jobs
Cleanup some devstack experimental jobs

-manila-tempest-...py35...centos7 : centos doesn't have python3
   support so these seem not required

-tempest-dsvm-multinode-full : we have grenade multinode in the check
    queue now, which would seem to cover this

-dsvm-cells : you can trigger this via the nova queue

Change-Id: I3b96e23b412ebf542f83a496bb1de91624a91380

openstack-infra/project-config,openstack-infra/project-config