only destroy single namespace if router_id is set

Fixes bug 1122206

If multiple instances of l3_agent are running on the same host, all qrouter-
namespaces will be destroyed as new l3_agents are started.  This fix allows
for multiple l3_agents to be running on the same host when router_id is set
for each agent.

Change-Id: I879cdc6faba94900f831232232d67e471c70d778