Updated entities.ActivationKey attribute.

Realized that the API for ``entities.ActivationKey`` uses
``max_content_host`` to limit its usage and not ``usage_limit`` as it
was previously thought.
Added new ActivationKey tests for API v2.

Added several new ActivationKey automated tests 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
```
Fixed typo on docstring.
Wrapped all ``entity()create()`` with try/except.

As per feedback from Jeremy, I have wrapped all ``entity()create()``
calls with a try/except block, as well as formatted the arguments with
it.
Merge pull request #1089 from omaciel/activationkey-v2-api

Activationkey v2 api