Validate that snat namespace exits in _create_dvr_gateway
Validate that snat namespace exits in _create_dvr_gateway

During rescheduling dvr router snat namespace may not be
created due to race between router added and router
updated updated notifications.

Verify that snat namespace exits or create one.

Partial-bug: 1881995

Change-Id: Ic28ce249d59264b0b882bd1cc3c9fb55854a6a47

openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron,openstack/neutron,mahak/neutron