Centralize neutron config file argument generation

Moves dynamic generation of the --config-file list from local functions
to global utility functions.  This breaks up the current non-obvious
dependency between start_neutron_agents and _configure_neutron_l3_agent
for setting the correct arguments for the vpn agent.   This also similarly
updates generation of arguments for neutron-server and neutron-l3-agent to
use said functions.  Finally, this cleans lib/neutron up a bit and moves
all default paths to config files and binaries out of local functions and
into the top-level, allowing external tools like Grenade to make use of the
library for starting Neutron services and agents currently.

Change-Id: I927dafca8a2047d6c0fd3c74569ed2521f124547
Closes-bug: #1355429
Merge "Centralize neutron config file argument generation"