Import extension from ceilometer

The original version of this extension was part of the
ceilometer code base at https://github.com/openstack/ceilometer.
It is being moved to its own package to make it easier to
reuse with other OpenStack components.

Change-Id: I9cfc2ee84c8b0a918dbcb59a8a5b8a3cb2bce044
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>