Remove centos-7 container for U

Since kolla has dropped py2 support in Usurri, we can should rely on the
centos-8 job from now on. This change makes the centos-7 job active only
for rocky, stien, and train.

Change-Id: I63784342843b540d48790601829da0159e1b50d9
Related-Bug: #1864018

zuul.d/build-containers.yaml

@@ -76,7 +76,7 @@
     name: tripleo-build-containers-centos-7
     parent: tripleo-build-containers-base-with-files
     nodeset: single-centos-7-node
-    branches: ^(?!stable/(pike|queens)).*$
+    branches: ^(stable/(rocky|stein|train)).*$
     voting: true
 
 - job: