Fix generate conf script can't handle multistropt

In case MultiStrOpt option's value is empty list, current extract_opts.py
logic will ignore it. This patch aims to update func _print_opt:

add the multistropt's option_name in conf even its value is empty list.

This bug also affects https://bugs.launchpad.net/cinder/+bug/1240783

Closes-bug: #1241348

Change-Id: Ica1da0cd20c97a587964cb0059836d609a440145
Merge "Fix generate conf script can't handle multistropt"