Potentially allow disabling the creation of the elastic user.

[naemono]

Currently the ECK Operator always creates the public-facing elastic user with superuser role for any Elasticsearch cluster. This issue is created to discuss if we should allow for this to be disabled on a per-cluster basis, and if so what would need to change for this to happen.

Known changes that would have to occur:

1. eck-diagnostics: This uses the elastic user by default. We would need to add the option of specifying a username/password combination to use that had sufficient privileges.
2. documentation: We reference this user in our docs. We would need to augment these to note that this user can be disabled.
3. e2e tests: We would likely want to add an e2e test that disables this user and verifies the lack of the k8s secret.

To discuss:

1. Any concerns for pursuing this?
2. Are there any further integrations that we are unaware of that would cause issues when disabling this user?