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
Merge "Refactor the stop operation for neutron-dhcp-agent." into stable/5.1