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.
If I strictly follow avoiding using single use variables, then it becomes:
```
self.assertIsNotNone(session.nav.wait_until_element(common_locators['has_error']))
```
Which atleast I find hard to read quickly. I feel, if it improves readability of the code then single use variables is fine. What do you think?