Move agent cleanup scripts to cmd module

Move agent cleanup scripts to cmd module

Ensure naming consistency across the various entry points.

This patch better separates active elements of Neutron agents from
operator's utilities.

Along with [1,2,3], this is done to better delineate what the agent
module inner boundaries are about. More to follow to finish off the
cleanup.

[1] https://review.openstack.org/#/c/146024/
[2] https://review.openstack.org/#/c/145979/
[3] https://review.openstack.org/#/c/146239/

Change-Id: I1d2d2938aa7fa1c60e0f0dc70997778a1930efef