gerrit-list: Add command-line arguments parser

* use argparse module to get host, port, username and project from
  command-line;
* add defaults to the options, suitable for review.openstack.org;
* if username is not specified and not in git remote, try to get it
  from git-review configuration.
Rename gerrit-list to gerda

Gerda and Gerrit are pretty much same name, only in different
countries and genders.
Turn gerda into toolset

* implement subcommands in command-line parser;
* turn list into one of subcommands;
* add 'dump-config' subcommand to make list not so lonely.