Adding optional delay between l3 agent restarts

This change serialises the neutron l3 agent restart process and adds a
user configurable delay between restarts. This can prevent connectivity
loss due to all agents being restarted at the same time.

Routers increase the recovery time, making this issue more prevalent.

Change-Id: I3be0ebfa12965e6ae32d1b5f13f8fd23c3f52b8c