Make System().create() work

When `System().create()` is called, it fails with the following error messages:

* `u'facts': [u"can't be blank"]`
* `u'cp_type': [u'is not included in the list']`

Update the `System` entity so that these errors are no longer encountered. To
fix these errors, the make the following fixes:

* By default, set `System.facts` to `{'uname.machine': 'unknown'}`.
* By default, set `System.type_` to 'system'.

Also:

* Add attribute `System.uuid` and override the default implementation of
  `System.path` to use a system's UUID instead of ID for identification.
* Alphabetically reorder `System`'s attributes.
* Remove the set of comments from `System`. These comments can be dangerous, as
  they can become outdated relative to the official API docs. (Note that this
  was the case with the `System.type` attribute.)
* Rename `System.system_type` to `System.type`. This is done so that the
  `System` entity can map as closely as possible to what the server is
  expecting.
Test the System entity

Add the `System` entity to module `tests.foreman.api.test_multiple_paths`.
Systems cannot be fully tested by this module, though, as the module expects
each entity to be uniquely identified by an ID, and a system is uniquely
identified by a UUID. Add module `tests.foreman.api.test_system_v2`, which makes
use of system UUIDs in its tests. For more details, see the docstring in module
`test_system_v2`.

Test method `System.path`.