Added test for activation key auto_attach

Added a test that updates an activation key's auto_attach field with the
inverse of its current value.
Also added a field to the ActivationKey entity for auto_attach.
```
@Test: Create an activation key, then update the auto_attach ... ok
```
Sure, but `assertNotEqual` is less accurate. This assertion succeeds:

    assertNotEqual(True, False)

These assertions also succeed:

    assertNotEqual(True, 'foo')
    assertNotEqual(True, 1)
    assertNotEqual(True, None)