Description of problem:
=======================
Fail to edit subnet CIDR and/or Gateway.
User can only edit subnet name

Version-Release number of selected component:
=============================================
5.7.0.12-rc2.20161123111642_a4a65ce

How reproducible:
=================
100%

Steps to Reproduce:
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1. Create a network on overcloud
2. Create a subnet on it 
3. Select the subnet click configuration and 'Edit selected Cloud Subnet'
4. Update Subnet Name, CIDR and Gateway and Save 

Actual results:
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Only subnet name update

Expected results:
=================
Gateway and CIDR should update too,

If CIDR Couldn't be update, the field should be grayed