Implement --downburst-conf parameter for teuthology-lock.
Load the appropriate yaml information when found (this formerly
did not work).  Make sure teuthology --lock works with a downburst
entry in the yaml files.  Document how this works in README.rst.

Fixes: #6921
Reviewed-by: Dan Mick
Merge branch 'teuthology-fix-downburst-yaml-wusui'