Add support for the concept of _namespace_

[rzvoncek]

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It is currently not possible to add two clusters of the same name into Reaper. They will clash with each other, or rather, I cannot tell what precisely will happen.

At any rate, for the purposes of monitoring and encryption, as well as usage in other projects, Reaper should be capable to work with equally named clusters. One way of doing this is to add a notion of a namespace, which will be a string optionally provided when adding the cluster to Reaper.

The exact requirements of this are still to be fleshed out.

┆Issue is synchronized with this Jira Story by Unito
┆Issue Number: REAP-195