packaging: setup: allow external plugins yum locking

Added infrastructure for allowing external plugins to
filter and add packages to version locking transaction.

Change-Id: Ic5a33d3a09e1c5aa9fed8a08d81a4ce583c12797
Bug-Url: https://bugzilla.redhat.com/1011087
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>