chore(role:pxc): Increase max_allowed_packet to 16M (#382)

The server's default max_allowed_packet value is 4MB in Mysql 5.7.
It is not enough for `kube_cluster_config` update in COE cluster update by heat.

fyi, Mysql 8.x uses 64MB as the default vault.

Co-authored-by: Mohammed Naser <mnaser@vexxhost.com>
diff --git a/roles/percona_xtradb_cluster/vars/main.yml b/roles/percona_xtradb_cluster/vars/main.yml
index 7692832..5721e0f 100644
--- a/roles/percona_xtradb_cluster/vars/main.yml
+++ b/roles/percona_xtradb_cluster/vars/main.yml
@@ -9,6 +9,7 @@
       [mysqld]
       max_connections=8192
       innodb_buffer_pool_size=4096M
+      max_allowed_packet=16M
     sidecars:
       - name: exporter
         image: "{{ atmosphere_images['prometheus_mysqld_exporter'] | vexxhost.kubernetes.docker_image('ref') }}"