arc/cli/args/parser.py: allow commands to have only short or long options

Before this, it was necessary for each option to have a short (-x) and
long (--extra) format.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
arc/cli/args/host.py: clean up options namespace

For actions used less frequently, let's just use a long option. This
also removes the "user experience bug" that I introduced by adding
'-L' as the short option for the standard "list-full" action.

Signed-off-by: Cleber Rosa <crosa@redhat.com>