Remove devstack-centos7 from nodepool
Remove devstack-centos7 from nodepool

We have no jobs using devstack-centos7 images any more, so we can
remove the images from nodepool.

Change-Id: I458bca80e45ae91099dd07bc38cfbf22174d36dd
Signed-off-by: Paul Belanger <e7139c8e6c4ea75cc1c287ce081b2dc0f3ab39ec@redhat.com>

Tesora/tesora-project-config,openstack-infra/project-config,Tesora/tesora-project-config,dongwenjuan/project-config,openstack-infra/project-config,dongwenjuan/project-config