Update docstrings
Update entity OperatingSystem

Update fields on entity OperatingSystem to reflect what the API expects. Add
comments that detail the reasoning for interesting changes.

Update IntegerField to accept optional minimum and maximum values. Add tests to
exercise the new code. This has been done to enable the necessary changes to the
OperatingSystem entity.
Add several tests for path /operatingsystems

The assertion in method `OperatingsystemsIdTestCase.test_post` fails when the
tests are run. This appears to be a bug, and a bug report should be filed.
Reduce the default max length of StringField

This is not done so as to be easier on the Foreman server. Rather, this is done
to make it easier to read through debugging output.
Add decorator skip_if_bz_bug_open to failing test
Make docstrings more meaningful
Merge pull request #963 from Ichimonji10/operatingsystems

Test `/operatingsystems`