Reduce duplication of options.

Many of the rbt commands use similar options. For example, any command that
talks to the server is going to want to have --server, --username, --password,
and --disable-proxy. In many cases, we had duplicates of these options, but
with slight differences and omissions scattered throughout.

This change moves the common OptionGroup definitions into the Command class,
and references those from each Command subclass's option_list.

Testing done: Ran rbt <command> --help for every command.

Reviewed at https://reviews.reviewboard.org/r/5506/