lib/neutron: configure root_helper for agents
lib/neutron: configure root_helper for agents

Before the patch, we were only configuring root_helper_daemon to point
to oslo.rootwrap, but not root_helper. (The former is used for long
running commands only, while the latter is used for short lived
commands.) This made neutron agents to directly call to sudo when a
privileged process was to be executed. This failed because /etc/sudoers
was not configured to allow anything except the rootwrap call itself.

This patch simplifies rootwrap handling in the code; it also sets
root_helper to point to rootwrap; as well as configure daemon in
sudoers. While at it, we also set l2 agent to use rootwrap too.
Hopefully, it will be enough for agents to actually configure backend as
needed.

Change-Id: Ib05a6e0e024f534d7f616d41d70fb67ecf6daeaf

dtroyer/devstack,noironetworks/devstack,nawawi/openstack,openstack-dev/devstack,group-policy/devstack,dtroyer/devstack,nawawi/openstack,group-policy/devstack,noironetworks/devstack,openstack-dev/devstack