config: added backward compatibilty

Added backward compatibility supporting upgrade from
3.1 version which used the keyword rhevm instead of engine.
When reading configurarion files, the user will be warned
that there is a deprecated key that needs to be updated.
(cherry-picked with modifications from commit
6ec3569d32d9b187f10cfa47e022d5d980432465)

Change-Id: Ic522c278157022b54d449bd4b0d73f9b9a2ce73d
Bug-Url: https://bugzilla.redhat.com/975417
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>