Update UI subnet tests

Instead of using a constant netmask, use the FauxFactory gen_netmask
generator.

This fixes `test_update_subnet_3`, the entire module test results:

```
$ py.test tests/foreman/ui/test_subnet.py
===== test session starts =====
platform darwin -- Python 2.7.5 -- py-1.4.24 -- pytest-2.6.2
collected 13 items

tests/foreman/ui/test_subnet.py .s.s.........

===== 11 passed, 2 skipped in 582.47 seconds =====
```