DBaaS Preview: Create DB cluster with randomly-generated password

User Story:

• As a PMM Admin user I need to get a random password for every DB cluster I'm creating using DBaaS so that increase the security of my instances

Suggested implementation:

1. Create a custom secrest resource like this where the password is a randomly generated and secure password.
The name should have the form of dbaas + db-cluster-name + db-cluster-type + secrets.yaml

apiVersion: v1
kind: Secret
metadata:
  name: cluster1-mongodb-secrets
type: Opaque
stringData:
  MONGODB_BACKUP_USER: backup
  MONGODB_BACKUP_PASSWORD: backup123456
  MONGODB_CLUSTER_ADMIN_USER: clusterAdmin
  MONGODB_CLUSTER_ADMIN_PASSWORD: clusterAdmin123456
  MONGODB_CLUSTER_MONITOR_USER: clusterMonitor
  MONGODB_CLUSTER_MONITOR_PASSWORD: clusterMonitor123456
  MONGODB_USER_ADMIN_USER: userAdmin
  MONGODB_USER_ADMIN_PASSWORD: userAdmin123456
  PMM_SERVER_USER: admin
  PMM_SERVER_PASSWORD: admin 

**2. kubectl apply - <secrets file from step 1>

3. Create DB cluster using the same name we use in the metadata: name section of the secrets file.  (cluster1-mongodb-secrets in this example)

4. Update the Get method  for PSMDB & PXC (example: https://github.com/percona-platform/dbaas-controller/blob/PMM-6899_cluster_external_connections/service/cluster/xtra_db_cluster.go#L220-L243) to retrieve secrets. Secrets can be retrieved from K8 with this command: 

kubectl get secrets my-cluster-name-secrets -o json  

—
UI:

Out of scope:

• The ability for the user to change or reset this password from  the UI 
• If the user changed it from the DB console  - it's ok to show the old one  here, until ^  implemented 

Acceptance Criteria:

• Password provided by the system for root user are randomized for every DB Cluster

How to test:

1. Create DB cluster
2. Call the GET method to retrieve the user info
3. Try to connect manually to cluster with credentials received in step 2