Use mongodb charm in k8s installer

- Update JUJU_AGENT_VERSION to 2.8.6
- Use mongodb charm and update needed osm pod specs to point at it

Change-Id: I7c03ae7de6611fcacef35716646ff08bfc1ff988
Signed-off-by: David Garcia <david.garcia@canonical.com>
diff --git a/installers/docker/osm_pla/pla.yaml b/installers/docker/osm_pla/pla.yaml
index c5bdd50..dfd00ab 100644
--- a/installers/docker/osm_pla/pla.yaml
+++ b/installers/docker/osm_pla/pla.yaml
@@ -41,6 +41,8 @@
           value: mongo
         - name: OSMPLA_MESSAGE_HOST
           value: kafka
+        - name: OSMPLA_DATABASE_URI
+          value: mongodb://mongodb-k8s:27017/?replicaSet=rs0
         volumeMounts:
         - name: osm-packages
           mountPath: /app/storage