Manually disable IP version when creating network subnets.

In some configuration, IPv6 is not supported. This patch provides
a way to disable IPv6 by hiding the ip version drop down menu and
setting the initial value to IPv4.

Closes-bug: 1352404

Change-Id: I950a1e19d509fdf4816816dd7b0f05bd418e7147

Conflicts:
	openstack_dashboard/dashboards/project/networks/tests.py
	openstack_dashboard/dashboards/project/networks/workflows.py

Implements: rally-task TA7024
Backported-from: Juno