The current listener API does not properly handle clearing/reseting
values on update. Some integer only fields, such as connection-limit,
will accept null, but will store the value as "None". These will
will cause failures updating the amphora configuration.
This patch corrects this to appropriately handle None/null updates
to the listener parameters.
Change-Id: I41c9bedd8a3452513af3d409fbacd65ea287f02a
Story: 2005374
Task: 30352
(cherry picked from commit 930a3236bf)
d086b3a98b