providers: add the onAddition and onRemoval hooks

Some providers need to perform additional operations on addition and
on removal, this patch gives the ability to the provider proxies to
override the onAddition and onRemoval methods in order to complete
the required actions.

Change-Id: I4b9a9af042ebd7988d90afeeecec4905a704894b
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>