Make Accumulo password secret optionally setable in values.yaml

[t92549]

Currently the Accumulo password secret is configured via the config.userManagement.rootPassword setting in the Accumulo values.yaml. This works well for dynamically created deployments, but if a user wants to save a repeatable deployment then this can become a security risk to store in plaintext. Instead, if the secret was optional, then a secret could be made once in the cluster and the repeatable deployment could be saved in plaintext without saving the password.