Some of the fields in subnet create page are not marked as required (with asterisk)

[Kahou Lei]

The subnet creation form doesn't indicate user which fields need to fill out and this will become problematic.

Steps to reproduce:
1. On the network panel, create a network.
2. Create a subnet.
3. In the Subnet modal form, since the first page doesn't have any require field indication, try to click next
4. User can't go to the Subnet detail page
The intend of not allowing user to go to the Subnet Detail page is because user hasn't filled out the Network field. Therefore, it is better to mark that as required.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/185114

[OpenStack Infra (hudson-openstack)]

Change abandoned by Kahou Lei (<email address hidden>) on branch: master
Review: https://review.openstack.org/185114

[Akihiro Motoki (amotoki)]

Fields reported here is *conditionally* required.
Thus adding 'required=True' (i.e., or dropping required=False as True is the default value) is not the right solution.
If you work on this, you might need to introduce some new mechanism to declare "this field is required when the field is displayed". I have no good idea at now though.