[py3] Let InstanceCommand.run_arg() always return an int
The caller of this method (which is method "run_args") does `max(status,
self.run_arg(appid))` and run_arg might return None if the underlying
<cmdname>_instance method returns None. On python3 max(1, None) breaks
with "TypeError: unorderable types: int() > NoneType()". So we enforce
run_arg() method to return an integer even if the underlying method
returns None.
This fixes call of "cubicweb-ctl i18ninstance" with Python3.
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile