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
```
Super minor thing, but this might be more readable:

```python
entities.ActivationKey().create(fields={
    u'unlimited_content_hosts': False,
    u'max_content_hosts': max_content_hosts,
})
```