Do not re-assign the bug if it is already assigned to the new point of contact
Update set_bugzilla_owner to accept the previous point of contact of the component

This is the only way we can track if the bugs are assigned to the new
point of contact, the old point of contact or neither.
Only update the user to which the bug is assigned when required

We update the person to which the bug is assigned only if we don't know
who was the previous point of contact or the point of contact is the one
that the bug is assigned to.

Fixes https://fedorahosted.org/fedora-infrastructure/ticket/3650