Immutable Parameters

[ANeumann82]

What would you like to be added:
While i'm on the parameter-feature-request spree: A flag for parameters to be immutable, i.e. they can only be specified on the installation of the operator and not changed with a kudo update.

This would probably be a flag in the params.yaml:

  - name: NUM_TOKENS
    description: "The number of tokens assigned to each node."
    default: "256"
    immutable: "true"

with a default of false.

Why is this needed:
There are certain things in an operator that by design need to be defined at installation time and can never be changed. The NUM_TOKENS for Cassandra for example is impossible to modify after the cluster is started.

See #1383 #1395

[ANeumann82]

Duplicate of #652