Update sphinx extension logging
Update sphinx extension logging

Sphinx 1.6 deprecated using the application object to perform logging
and it will be removed in the upcoming 2.0 release. This updates our
extensions to use the recommended sphinx.util.logging instead.

Change-Id: I49f55758b65e358154f3afbff3c47d1857073a4a
Signed-off-by: Sean McGinnis <961eeb09b6d45516665e15b82e013282c4faf575@gmail.com>

openstack/reno