Add defaults for config-dir

If no --config-dir switches are given on the command
line, use default directories to search for config
snippets.
This is similar to the default config-file support
oslo.config already includes. It is useful in environments
where command line arguments can not easily be added, like
mod_wsgi Apache envs.

Change-Id: I4df977911539777d1510e8b579375aca5b5f15f4