Refactor the stop operation for neutron-dhcp-agent

* Combine the stopping of neutron-dhcp-agent and the stopping of dnsmasq
processes inside namespaces into one time period (it was only for stopping
of ONE neutron dhcp process).
* Clean up duplicate code .

Change-Id: Iaaa2b6f7c8e3b0fa870715c69fd39fa45e5dc526
Closes-Bug: 1377906