provider: add the onModification hook

Change-Id: I8ed9ef0f798727fbd543cd409d91653841a3a27d
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
core: generalize image list messages

Change-Id: I3f028188993624aed6bff6179d406f0be03f3e1e
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
core: remove cleared tasks from _listTasks

Change-Id: Ibbbafbb9516c731c263ec1a5ac055705022829cf
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>