Revert "Refactor the stop operation for neutron-l3-agent" due to the fix not yet merged to master branch

This reverts commit 5032d9550627001ed9e9ccad94b37246e9fb9a11.

Change-Id: I5d24f811a17a0d916c330b04fea163d40ca57ab6
Merge "Revert "Refactor the stop operation for neutron-l3-agent" due to the fix not yet merged to master branch" into stable/5.1