Add --dc-suffix to create subcommand

[a1exsh]

This makes it consistent with extend.

[a1exsh]

:+1:

[slitsche]

I'd expect the value to be used in generate_taupage_user_data. But could not find it.

[a1exsh]

Not really, because that user data is a template, to which instance-specific details, such as volume name (which indeed depends on the dc_suffix), are added later: https://github.com/zalando-stups/planb-cassandra/blob/master/planb/create_cluster.py#L496-L509

[a1exsh]

:+1:

[slitsche]

👍