[RBAC] Add role binging for prometheus-server

In case of using RBAC in k8s we need to create role binding for
prometheus-server user to unlock creation of kubernetes-api target in
prometheus, otherwise we will get error:
`User \"prometheus-server\" cannot list nodes at the cluster scope`

Change-Id: I7ec27483f16169a5040aad24c9076c6361e02127
diff --git a/tests/pillar/master_cluster.sls b/tests/pillar/master_cluster.sls
index a75c173..a0c2a2c 100644
--- a/tests/pillar/master_cluster.sls
+++ b/tests/pillar/master_cluster.sls
@@ -43,6 +43,8 @@
         - cmp01
         - cmp02
         image: mirantis/virtlet:v0.8.0
+    monitoring:
+      backend: prometheus
   master:
     admin:
       password: password