do not setup bridge ip if bridgeNetwork is nil. This is the case when
-b='none' bridge is provided.

issue #2768
https://bugzilla.redhat.com/show_bug.cgi?id=1032094
fix the nil pointer panic on closing a disabled network manager
Issue #2768
Merge pull request #2769 from vbatts/none_bridge

Do not setup bridge ip if 'none' bridge is provided