1139703 - update pickled schedule on schedule updates

Previously, the pickled CelerySchedule was being set on create but not update.

This patch checks to see if `iso_schedule` is being updated, and pickles an
updated CelerySchedule object to match the new `iso_schedule`.
1142304 - remove extraneous errors during unit test runs

A few areas of the Pulp code used  `__del__` to ensure some cleanup tasks were
completed. However, `__del__` can be called during garbage collection [while
cPython is doing cleanup][del], which results in error messages being
printed to stderr. These messages are ultimately harmless since they occur
during instance teardown. However, we can remove these calls to `__del__` and
rely on other mechanisms instead.

In the *UnitWriter* class, the `__exit__` method will always invoked since the
class is only called by a `with` statement. This is performing the same cleanup
function as the `__del__` method.

In the *Certificate* class, we can do temp dir cleanup directly inside the
`finally` block.

This patch removes the `__del__` methods in these classes and relies on the
aforementioned methods along with regular garbage collection to do cleanup.
When I ran GC stats after making these changes, I didn't see any unaccounted
references.

[del]: https://docs.python.org/2/reference/datamodel.html#object.__del__
Merge pull request #1171 from beav/__del__

1142304 - remove extraneous errors during unit test runs
Merge remote-tracking branch 'origin/2.5-dev'
Merge pull request #1170 from beav/pickled-schedule

1139703 - update pickled schedule on schedule updates
Merge remote-tracking branch 'origin/2.5-dev'
add release note for pulp_docker
Merge pull request #1181 from beav/docker_relnote

add release note for pulp_docker
Merge remote-tracking branch 'origin/2.5-testing' into 2.5-dev
Merge remote-tracking branch 'origin/2.5-dev'