Support dc name overrides in cass-operator

[adejanovski]

cass-operator recently got a new feature which allows to override a Cassandra DC name with a value that's different than the cassdc object name. This allows creating Cassandra DCs with names that aren't valid kubernetes names, with spaces, uppercase characters, ...

We need to expose this feature in k8ssandra-operator through a new DatacenterOptions.DatacenterName field which will only be considered at the DC level, and not the global level (obviously).