engine: wrong log severity for interface up/down event logs

Interface down events' log severity should be warning.
Interface up events' log severity should be normal.

Change-Id: I9ff6b663a141f75241ff27f8df12922c42de04ff
Bug-Url: https://bugzilla.redhat.com/1113160
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>