Remove the fixed set of choices for network_type
Remove the fixed set of choices for network_type

So that it can specify out-of-tree ML2 type drivers.

Closes-Bug: #1657311
Change-Id: I2445fb165b86cf5937d8aa09ba0fd5564eb1f8f4

openstack/python-openstackclient,dtroyer/python-openstackclient,openstack/python-openstackclient,dtroyer/python-openstackclient