Optionally delete namespaces when they are no longer needed

Adds a configuration option to tell the network agents to delete
namespaces when they are no longer in use.  The option defaults to
False so that the agent will not attempt to delete namespaces in
environments where this is not safe.

This has been working well in deployments where iproute2 has been
patched with commit 58a3e8270fe72f8ed92687d3a3132c2a708582dd or it is
new enough to include it without being patched.

Change-Id: Ice5242c6f0446d16aaaa7ee353d674310297ef72
Closes-Bug: #1250596
Related-Bug: #1052535
(cherry picked from commit 7336f3bd27d138b3d11d601f977a1e3df2a44b3e)
Related-Bug: #1175695
Partial-rally-bug: DE745
Upstream-review: https://review.openstack.org/#/c/84419/