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
7a0f6902576b1f71aa844f592c03d0514976f841)

Change-Id: I6755c1ad020ee3220aa5f5335c6d66ead5139e57
Bug-Url: https://bugzilla.redhat.com/975416
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>