Fix up `zuul --version`

argparse does not support optional subparser, hence the `zuul` client
command would always require to be passed a sub command.  That prevented
`zuul --version` from showing up with a 'too few arguments' error.

I found out add_argument() has a action='version' which would print the
content of the 'version' argument and exit.

Change-Id: I553d3b2ab7aa04865e44559135adc5a7640307ed
Merge "Fix up `zuul --version`"