Activationkey v2 api

Added a few automated tests for ``entity.ActivationKey`` using the API v2.

```bash
$ nosetests -c robottelo.properties tests/foreman/api/test_activationkey_v2.py
nose.config: INFO: Ignoring files matching ['^\\.', '^_', '^setup\\.py$']
@Test Create activationkey with limited content hosts but no limit set ... ok
@Test Create activationkey with limited content hosts but ... ok
@Test Create activationkey with limited content hosts but ... ok
@Test Create activationkey with limited content hosts but ... ok
@Test Create activationkey then update its max content hosts ... ok
@Test Create activationkey then update its max content hosts ... ok
@Test Create a plain vanilla activationkey ... ok
@Test Create an activationkey with limited content hosts ... ok
@Test Create an activationkey with limited content hosts ... ok
@Test Create activationkey with unlimited content hosts and set ... ok
@Test Create activationkey with unlimited content hosts and set ... ok
@Test Create activationkey with unlimited content hosts and set ... ok
@Test Create an activationkey providing the initial name. ... ok
@Test Create an activationkey providing the initial name. ... ok
@Test Create an activationkey providing the initial name. ... ok
@Test Create an activationkey providing the initial name. ... ok
@Test Create activationkey then update it to limited content hosts ... ok
@Test Create activationkey then update it to limited content hosts ... ok

----------------------------------------------------------------------
XML: ~/hacking/robottelo/foreman-results.xml
----------------------------------------------------------------------
Ran 18 tests in 68.013s

OK
```
This can be trimmed down. How about this?

```python
attrs = entities.ActivationKey().create(fields={u'name': name})
```

If the creation fails, a `FactoryException` is raised. You could thus do this instead:

```python
try:
    attrs = entities.ActivationKey().create(fields={u'name': name})
except factory.FactoryError as err:
    self.fail(err)
```