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.
Merge pull request #1403 from Ichimonji10/test-user

Convert old API tests for users
Org & Loc tests UI: Dependent Entities use now APIs

The dependent entities now use APIs to create them.

Now `ui/test_location` and `ui/test_org` now uses `FauxFactory`
module for generate_string, generate_ipaddr, generate_email.

Fixed the organization and location tests as per comments.

Few Fixes after the rebase
Merge pull request #1394 from kbidarkar/update-location-ui-tests

Org & Loc tests UI: Dependent Entities use now APIs
Completed cli fencing.  There may be same code changes required for some functions in future though
Merge pull request #1409 from sthirugn/moreclifence

Completed cli fencing.  There may be same code changes required for some...
Added content view test adding filter to rh content

Closes #1400
Merge branch 'AdamSaleh-content-view-rh-associate'
docstrings formating and key unicoding
Merge pull request #1414 from sghai/fix-docstring-unicodes

Fixed disctrings formating and unicode all keys under UI factory