The current openstackclient implementation cannot accept 0
for --min-disk and --min-ram with the "openstack image set" command.
If theses options get set to 0, the option parser in openstackclient
wrongly interprets 0 as no option value. The 0 is valid for these
options if administrators want to make it the default(no minimum
requirements).
This patch fix the parser so that it avoids only 'None'.
Change-Id: Ie8ee37484c02c26f54adc56263fcd167c0ce7eb3
Closes-bug: #1719499
4464109c77