Add API tests for subnets

Make it possible to create, read and delete subnets using the `Subnet` class in
module `robottelo.entities`. Do this by adding several mixins to that class.
Verify that it is possible to create, read, update and delete subnets by adding
them to module `test_multiple_paths`.

Remove module `test_subnet`. It contains nothing but test stubs.
Merge pull request #1395 from Ichimonji10/test-subnet

Add API tests for subnets
Convert old API tests for users

Delete module `test_users`. Rewrite module `test_user_v2`, and drop in a test
that thoroughly tests whether it is possible to create and delete a user with a
variety of attributes.

Update the definition of the `User` class. Set some better limits on how long
the fields `firstname`, `lastname` and `login` can be.