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.

Ran rbt <command> --help for every command.