L3 Agent restart causes network outage

When a L3 agent controlling multiple qrouter namespaces
restarts, it destroys all qrouter namespaces even if
some of them are still in use.  As a result, network
traffic could be stopped on the VMs that use the
networks associated with these namespaces.

So what is needed is for the L3 agent to preserve those
qrouter namespaces a L3 agent instance recognizes and to
destroy those it does not know about.

Closes-Bug: #1175695

Change-Id: Idae77886bd195d773878c3d212ccfd56269216fb
(cherry picked from commit 011d99f300ea5d5f4ce48023bd04a795a4872287)

Backported-from: icehouse
Closes-rally-bug: DE833
Upstream-Review: https://review.openstack.org/#/c/84420/