Set proper yum repo priority on CentOS 7

This patch ensures that we get galera from the MariaDB repos
and not from RDO. This matches I2c8e4e64c2425cd36903ae9288bd9fee29eef355
which did the same thing for the galera-server role.

Closes-Bug: #1739472
Change-Id: I8f22083a306ba7569148a53af94397c31d90dbac
(cherry picked from commit 934344ab41)

tasks/galera_client_install_yum.yml

@@ -86,3 +86,13 @@
     - "{{ galera_client_repo }}"
   tags:
     - galera-repos
+
+# NOTE(hwoarang): MariaDB repository is prioritized at 99 by default
+# and that allows yum to install mariadb client from the RDO repos, which
+# is not good. This task ensures that the following tasks will choose
+# MariaDB repo as the highest priority.
+- name: Ensure MariaDB repository takes highest priority
+  command: |
+      yum-config-manager
+        --enable MariaDB
+        --setopt="MariaDB.priority=25"