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
Re-order stack.sh 1: sanity checks

Part 1 of a series

Re-order the setup and check bits in the top portion of stack.sh to
have a logical flow with similar things done together.

No behaviour changes are intended aside from the order of execution.
Any such changes are bugs.

* Do sanity checks that have no configuration needs earlier
* Do supported distro check earlier

Change-Id: I7d15bac199d6c4382d4a4d222784d34f2707da56
Change httpd Keystone log level to default

Having Keystone's log level at debug caused a lot of uninteresting
Httpd-related log lines to be displayed which makes debugging more
difficult than it should be.

Rather than set the log level explicitly, Keystone will use the
Httpd server's setting, which defaults to warn.

Partial-Bug: #1359995
Change-Id: Ieef882944bafd98f7b27497a5276124b21c3e576
Merge "Centralize neutron config file argument generation"
Merge "Change httpd Keystone log level to default"
Merge "Re-order stack.sh 1: sanity checks"
Merge branch 'master' of https://github.com/openstack-dev/devstack