engine: adding DefaultMTU config value

DefaultMTU will be used as an oVirt-level default.
The concept of "MTU: Host's default" is eliminated.

Change-Id: Ifb7a50da7e77e1eeb27a5fabb48aafd7dcf6496f
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>