Add a separate scylla node as the manager's backend

[ShlomiBalalis]

Currently, the scylla manager uses the same cluster that is used for testing as a backend. This creates a problematic situation on certain test cases. Can't we create a separate scylla cluster that will be used as the backend?

[ShlomiBalalis]

@slivne

[fruch]

@mikliapko is that something of interest for you, or we can close it ?