Update MongoDB connection string to use k8s service DNS

[asafchen-dig]

This PR updates the MongoDB connection string to use the Kubernetes service DNS name for petclinic-flaskdb service.

The petclinic-flaskdb deployment manifest should be created in a separate PR due to some technical limitations with file creation.

[asafchen-dig]

Due to technical limitations with file creation, the deployment manifest for petclinic-flaskdb needs to be created manually. Here's the content that should be added in a new file (e.g., deploy/petclinic-flaskdb.yaml):

apiVersion: apps/v1
kind: Deployment
metadata:
  name: petclinic-flaskdb
  labels:
    app: petclinic-flaskdb
spec:
  replicas: 1
  selector:
    matchLabels:
      app: petclinic-flaskdb
  template:
    metadata:
      labels:
        app: petclinic-flaskdb
    spec:
      containers:
      - name: mongodb
        image: mongo:4.4
        ports:
        - containerPort: 27017
        env:
        - name: MONGO_INITDB_DATABASE
          value: "petclinic"
---
apiVersion: v1
kind: Service
metadata:
  name: petclinic-flaskdb
spec:
  selector:
    app: petclinic-flaskdb
  ports:
  - port: 27017
    targetPort: 27017
  type: ClusterIP

This manifest will create:

1. A MongoDB deployment running version 4.4
2. A Kubernetes service named petclinic-flaskdb that exposes port 27017
3. The service uses the same name that we configured in the application.properties