[RFE] Entering an empty content in Subnet fields does not generate an error

[Liron Kuchlani]

Description of problem:
Entering an empty content in the fields of 'Create Subnet' window, does not generate an error, indicating the necessity to fill the fields

Version-Release number of selected component (if applicable):
python-django-horizon-2015.1.0-10.el7ost.noarch

How reproducible:
100%

Steps to Reproduce:
1. Create a Subnet without empty content in the fields
2. Click 'NEXT'

Actual results:
Nothing happens

Expected results:
The empty fields should be highlighted in red and an error should be appeared, indicating the necessity to fill the fields