Add map_method function to managers

This allows to use map() directly over some extension method in a more
convenient way. It's a pattern we often need, so let's built it directly
in Stevedore.
Merge pull request #27 from jd/master

Add map_method function to managers
Fix flake8 failures from pull/27

Fix a few formatting issues from within the
previous pull request.

Change-Id: Iadb0d5d4bfa1ac460c730e8af0cfec9386a4c066