[DM]: Do not assume vmi device_owner attribute is always set by api clients
[DM]: Do not assume vmi device_owner attribute is always set by api clients

If not set, set it to ''. Also alow to have device owner attribute value can be PhysicalRouter or physical-router

Change-Id: I2ebef4d16ae8739a4d8a63b125b4aa880fb7d58e
Partial-Bug: #1646684

nischalsheth/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller