Switch nova-docker-f20 job to centos7

F20 nodes are having issues at the moment, and F20 is getting a bit
old anyway.  Move this to centos7 which is a better choice for
long-term testing.

Change-Id: Ifa742ba5bdae0b796b8b92336e41234fd8b7939e

jenkins/jobs/nova-docker.yaml

@@ -47,8 +47,8 @@
       - console-log
 
 - job:
-    name: 'check-tempest-dsvm-f20-docker'
-    node: 'devstack-f20'
+    name: 'check-tempest-dsvm-docker-centos7'
+    node: 'devstack-centos7'
 
     wrappers:
       - build-timeout:

zuul/layout.yaml

@@ -895,7 +895,7 @@ jobs:
     voting: false
   - name: check-tempest-dsvm-docker
     voting: false
-  - name: check-tempest-dsvm-f20-docker
+  - name: check-tempest-dsvm-docker-centos7
     voting: false
   - name: check-neutron-dsvm-api
     branch: ^(?!stable/(icehouse|juno)).*$
@@ -5587,7 +5587,7 @@ projects:
       - name: python-jobs
     check:
       - check-tempest-dsvm-docker
-      - check-tempest-dsvm-f20-docker
+      - check-tempest-dsvm-docker-centos7
     post:
       - nova-docker-coverage