Description
===========
If I set min-disk or min-ram to a value greater than 0 with "openstack image set" command once, I can't reset them to 0 unless I recreate the image.
It's very inconvenient.

Steps to reproduce
==================
1. Create an image.
   #openstack image create --file <file> <image-name>
   +--- ------- ------- -+----- ------- ------- ------- ------- ------- ------- ------- +
   | Field | Value |
   +--- ------- ------- -+----- ------- ------- ------- ------- ------- ------- ------- +
  (snip)
   | min_disk | 0 |
   | min_ram | 0 |
  (snip)
   +--- ------- ------- -+----- ------- ------- ------- ------- ------- ------- ------- +
2. Update the image with "--min-disk 1" and "--min-ram 1".
   # openstack image set --min-disk 1 --min-ram 1 <image>
3. Show the image.
   # openstack image show <image>
   +--- ------- ------- -+----- ------- ------- ------- ------- ------- ------- ------- +
   | Field | Value |
   +--- ------- ------- -+----- ------- ------- ------- ------- ------- ------- ------- +
  (snip)
   | min_disk | 1 |
   | min_ram | 1 |
  (snip)
   +--- ------- ------- -+----- ------- ------- ------- ------- ------- ------- ------- +
4. Update the image with "--min-disk 0" and "--min-ram 0".
   # openstack image set --min-disk 0 --min-ram 0 <image>
5. Show the image.
   # openstack image show <image>
   +--- ------- ------- -+----- ------- ------- ------- ------- ------- ------- ------- +
   | Field | Value |
   +--- ------- ------- -+----- ------- ------- ------- ------- ------- ------- ------- +
  (snip)
   | min_disk | 1 |<--- min_disk wasn't changed
   | min_ram | 1 |<--- min_ram wasn't changed
  (snip)