use stevedore to load extensions
use stevedore to load extensions

Importing pkg_resources causes the app to scan the entire import path
for all distributions, not just those providing entry points. The
scanner in stevedore will have a cache of the entry point data, making
it significantly faster.

This also more accurately reflects the way oslo.config loads option
plugins.

Change-Id: Ibbac3240a99684de0dea2f690f4276a3d24e3fe1
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack/glance_store,openstack/glance_store