cli: Use list of options shared between commands

The completion script for bash has to know about these options. Until now
the list was in two places--once in cli.py and once in
autotools/build-bash-completion. A shared list is used with this patch.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>