Fixed many subnet related test cases.

Assertion for `test_create_subnet_3` depends on whether checkbox
or selection list exists.

Fixed many other miscellaneous test cases.
See [this comment](https://github.com/SatelliteQE/robottelo/pull/1795/files#r22108723).

> This change was made because the code is unclear, right? If so, I think a comment is the best solution: