EVEREST-1511 | support for deploying default DB namespace during installation

[mayankshah1607]

This PR brings 2 big improvements:

1. Deploy a default DB namespace along with the core chart installation:

• By default, the everest DB namespace will be deployed along with the core chart installation
• The default DB namespace may be skipped from the core chart by specifying dbNamespace.enabled=false
• The default DB namespace name may be overriden by specifying dbNamespace.namespaceOverride
• By default, all operators are installed. Specific operators may be skipped by specifying pxc|pg|psmdb=false

2. No longer need to manually create the namespace object before installing DB namespace

• Earlier you were required to create the namespace beforehand, so that we could attach the app.kubernetes.io/managed-by=everest label
• With this PR, the labelling happens automatically as a part of the post-install hook. So you no longer need to create a namespace manually.

See the updated README for full info.