Add the feature in cass-operator to provision user permissions(RBAC) through CRD file

[sync-by-unito]

In datastax/cass-operator#94 there is a function to define users through the CRD file, e.g

  users:
    - secretName: <a normal username>
       superuser: false

It is good to have the RBAC permissions associated with the users/roles so that end-users do not have to log in the cluster with superuser and do it manually.

e.g in CRD file, it can be like:

  users:
    - secretName: <a normal username>
       superuser: false
       permissions: create, drop on keyspace, table

┆Issue is synchronized with this Jira Story by Unito
┆Issue Number: CASS-57

[jsanda]

I think this is an excellent idea. Note that the design being proposed for k8ssandra/k8ssandra-operator#556 will impact this.