[Fix] Add Prometheus mysql exporter label to network policy

Prometheus mysql exporter is failing because of missing label in
network policy for mysql. This is to add the label to the override
yaml

Change-Id: Ic5e2613303f04d2bfab1229d3818a2fc9b4263bc
Signed-off-by: Sreejith Punnapuzha <Sreejith.Punnapuzha@outlook.com>

tools/deployment/developer/common/050-mariadb.sh

@@ -75,6 +75,9 @@ network_policy:
         - podSelector:
             matchLabels:
               application: mariadb-backup
+        - podSelector:
+            matchLabels:
+              application: prometheus-mysql-exporter
         ports:
         - protocol: TCP
           port: 3306