Fix error while connecting to busy NSX L2 Gateway

Ensure that a Conflict error is properly handled.

Change-Id: Ib468ee953aef0fc54ea88064f88637ed46bc6352
Closes-bug: 1270724
Use oslo.rootwrap library instead of local copy

Remove rootwrap code copied from oslo-incubator, make the
{neutron,quantum}-rootwrap console_script entrypoints point to
oslo.rootwrap code instead.

Adjust bin/{neutron,quantum}-rootwrap[-xen-dom0] so that it calls
into oslo.rootwrap.cmd.

Change-Id: I22df4060d6bca6affd7761fec49d2767ca8f59cf
Implements: blueprint neutron-oslo-rootwrap
Merge "Use oslo.rootwrap library instead of local copy"
Merge "Fix error while connecting to busy NSX L2 Gateway"