Put admin functions in _proxy_admin

[Everett Toews]

For any service that exposes admin functions, we should put those functions in a separate file _proxy_admin and expose it in a separate dynamically loaded module. Perhaps we don't even load that module if the user doesn't have admin privileges. For example,

    conn.compute_admin.create_flavor()

The focus of this SDK is end user developers, not administrators. Mixing user functions and admin functions is at best confusing and at worst a source of bugs for users who don't realize they're attempting to do admin functions.

Most admins will not use this SDK directly as they typically aren't coding in Python but rather in bash or the like. Admins will mostly be using this SDK indirectly via OpenStack Client, Ansible, or some other higher level tool. They will get no direct benefit from having their functions mixed in with everything else.

We do need to support admin functions but let's keep them separate to minimize confusion.