Validate existence of DCs when repairing

With the new validation of data centers in the replication factor, it might be good to give similar treatment to repair.

Currently the behavior of the --in-dc flag only validates that it contains the local data center.

If a list is given containing nonexistent data centers, the repair will pass without errors or warning as long as this list also contains the local data center.

My suggestion would be to validate all the data centers and give an error when a nonexistent data center is given.