Allow set_default and set_override to use None

The current implementation interprets set_default('foo', None) and
set_override('foo', None) as 'clear the existing default or override',
which makes it impossible to override a value with None.

This patch adds support for overriding with a None value by adding
a special internal class. set_override('foo', None) will now override
the existing value with None. This is a slight change to the existing
behavior, so this patch adds two calls for the old functionality of
clearing defaults and overrides. Example syntax for the new calls
are shown below:

conf.clear_default('foo')
conf.clear_override('foo')

The patch also updates the tests to reflect the change in functionality
and adds new tests to verify the new functionality.

Fixes bug 1035478

Change-Id: Iee5e20e44da9bef6b86e0483ab0b48b625fe503c

0 files changed, 0 insertions, 0 deletions