Migrate CentOS jobs to ubuntu bionic beaver

Jobs for manila proper have migrated from CentOS to
ubuntu bionic beaver so that we can run them under
python 3.

Make corresponding changes to the zuul configuration
for manila-tempest-tests.

Change-Id: Ic63aaa87b5980436d61bc57c957149e20c4b8bd2
diff --git a/.zuul.yaml b/.zuul.yaml
index 1e44daf..4c95c89 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -16,17 +16,17 @@
             voting: false
         - manila-tempest-dsvm-scenario:
             voting: false
-        - manila-tempest-minimal-dsvm-cephfs-native-centos-7:
+        - manila-tempest-minimal-dsvm-cephfs-native:
             voting: false
-        - manila-tempest-minimal-dsvm-cephfs-nfs-centos-7:
+        - manila-tempest-minimal-dsvm-cephfs-nfs:
             voting: false
         - manila-tempest-minimal-dsvm-dummy
-        - manila-tempest-minimal-dsvm-lvm-centos-7
+        - manila-tempest-minimal-dsvm-lvm
     gate:
       queue: manila
       jobs:
         - manila-tempest-minimal-dsvm-dummy
-        - manila-tempest-minimal-dsvm-lvm-centos-7
+        - manila-tempest-minimal-dsvm-lvm
     experimental:
       jobs:
         - manila-tempest-dsvm-glusterfs-nfs: