engine: Change backward compatibility logic for add/update vnic

The current backward compatibility logic designed for
resolving a vnic profile id for a given network should
be modified, since REST clients which provides both
network name and vnic profile id might face issues, such
ignoring the vnic profile id.

For that purpose the engine will attempt to find a change
in the network name provided by the user to what is already
configured on the vnic:

1. If network name is provided, and unchanged - the vnic
    profile id from the parameters is used.
2. If the network name is provided and changed, use it
    instead of the vnic profile id.

Change-Id: If1611087bad26b0bfa5a42780c5362e3413fbb75
Bug-Url: https://bugzilla.redhat.com/1064823
Signed-off-by: Moti Asayag <masayag@redhat.com>