Allow L3 and DHCP agents to cleanup system resources which they don't host anymore

Currently there is a rescheduling mechanism that allows L3 routers and DHCP services to be moved between agents when
neutron-server thinks particular agent is dead.
That however may lead to several that better be solved:

1) lots of unused namespaces on the network controller.
Complicates troubleshooting
2) running processes: dnsmasq or ns-metadata-proxies
Unmanaged dnsmasq could really do harm to the tenant network conflicting with dnsmasq with host file wich is up-to-date
3) waste of DHCP port

etc

These issues negatively affect cloud more and more as number of networks/routers grow.

The main idea of proposal to create a mechanism that would cleanup resources on agent so only those resources which are bound to the agent have physical representation.
Other physical resources (processes, interfaces, namespaces) are deleted.