Expose an entry point to list all config options

Register a oslo.db entry point in the oslo.config.options namespace
which, when called, returns a list of the configuration options which
may be registered by the library at runtime.

The idea here is that the sample config file generator can query this
and include the returned options in the sample config file of any
 applications which use the library.
Merge pull request #4 from malor/add_entry_point

Expose an entry point to list all config options