Make our conf files more deployer friendly

This is a combination blueprint to hold the various recommendations I've heard from deployers about how painful dealing with novas configs are. It may be split into more blueprints in the future. Here are the steps as I see them.

1. Stop using api-paste.ini for operational configuration. Anything we want to do with pipelines should be done via simple config parameters in nova.conf:
auth_strategy=keystone
keystone_user=xxxx
keystone_password=xxx

2. Add a conf.d directory. It makes it far easier to manage configurations via config management tools like chef when you can clobber a whole file based on installation options rather than having to templatize parameters in one file. This probably could be added to nova-common

3. Allow runtime config changes. The initial version could be just polling the configs for changes. This could also be added to nova-common

4. Centralized configuration. Either shared config overrides in the db, or a central server to pass common configs to all of the hosts in the system. This one needs to be investigated. We don't want to become another chef or puppet, but distributing common configurations to all hosts could be valuable.

5. Cleanup of config options. We have too many config options. Some of these can probably be changed to constants because they never reasonably need to be changed by a deployer. Some can be removed and simplified, and some are just unused.