Add option --node-tags to tell hroller to consider only nodes
with these tags. A use case would be a tag tracking on which
nodes the maintenance has not yet been carried out, e.g., if
rolling reboots are interleaved with other cluster operations.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>