Fixed post_network_plug issues
Fixed post_network_plug issues

If there was no interface to raise, it would insert bad data into
the interfaces file.  Also, execute ifdown first before ifup for
sanity that prevents some inconsistent successes and failures.

Change-Id: I96d552e0f46ebbdfd9827b9b9c818e5c561699b1

openstack/octavia,openstack/octavia,openstack/octavia