Hides prefix_length column in subnet show output
Hides prefix_length column in subnet show output

When the openstacksdk is patched to properly support defining prefix
lengths when creating subnets, the resulting subnet show output
reveals a prefix_length column with a value of 'none'. This patch
hides the prefix_length column.

Change-Id: I59dfb0b1585ed624f9d82b3557df2ff5ff9d1b3e
Partial-Bug: 1754062
Depends-On: https://review.openstack.org/#/c/550558/

openstack/python-openstackclient,openstack/python-openstackclient