set remote_cidr and remote_sg_id to None instead of empty. Also return NetworkInUse when network_delete fails
Fetch virtual_machine_interface_back_refs along with net props+refs to save on extra round-trip
Merge pull request #12 from Juniper/ajay_no_vmi_backrefs

Backport contrail/havana (4256f09ddaef5239b489d61ce8ddaca44eea524e) + Read VMI backrefs with network in one-shot.