Helm chart support mysql

[canghai118]

We are deploying superset use helm charts, in the current version of helm chart, Postgresql is hard coded in config files. It is inconvenient for user to configure an external mysql database for superset metastore. I think we should change the helm chart to support other database type, include mysql and sqlite

https://github.com/apache/superset/blob/master/helm/superset/templates/_helpers.tpl#L82

[jimyag]

configOverrides:
  mysql: |
    SQLALCHEMY_DATABASE_URI = 'mysql+pymysql://username:password@mysqlhostname:3306/superset'
....

postgresql:
  # disable pg
  enabled: false

supersetNode:
  connections:
        db_host: "mysqlhostname"
        db_port: "3306"
        db_user: username
        db_pass: password
        db_name: superset
bootstrapScript：|
  pip install PyMySQL