Logging via the application object was deprecated in sphinx 1.6 and is
now removed in the master branch that will become 2.0. This updates our
sphinx extension to use the recommended sphinx.util.logging instead.
Closes-bug: #1798174
Change-Id: Ie66579146d68680905c3eac3d256369309130cf4
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>