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.
Mind you, this is well above 79 characters:

```python
self.assertIsNotNone(session.nav.wait_until_element(common_locators['has_error']))
```

So, if all single-use variables are dropped, it would be written something like this:

```python
self.assertIsNotNone(
    session.nav.wait_until_element(common_locators['has_error'])
)
```

Which is much easier to read. But if you think that something like this is nicer, then OK. There's no functional problem with the code:

```python
locator = ...
self.assertIsNotNone(session.nav.wait_until_element(locator))
```

Use your best judgment. I think we've both taken a good hard look at this code by now, and I know you can come up with some good-looking code that uses some better names. :)