Fix error while connecting to busy NSX L2 Gateway

Ensure that a Conflict error is properly handled.

Change-Id: Ib468ee953aef0fc54ea88064f88637ed46bc6352
Closes-bug: 1270724
(cherry picked from commit 7ee1344c474b73dd2236d2c79df8dcf49cdb7bef)
Merge "Fix error while connecting to busy NSX L2 Gateway" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable