Refactor the stop operation for neutron-l3-agent

* Combine the stopping of neutron-l3-agent and the stopping of metadata
processes inside namespaces into one time period (it was only for
stopping of ONE neutron l3 process).
* Some cleanups and corrections.

Change-Id: I51a706acb3b1d4d3f0e12ef07577e7852152888f
Closes-Bug: #1391881
Merge "Refactor the stop operation for neutron-l3-agent" into stable/5.1