[recipes.py] make `--properties-file -` work correctly again.
[recipes.py] make `--properties-file -` work correctly again.

R=dnj@chromium.org, nodir@chromium.org, phajdan.jr@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2838923003

luci/recipes-py,luci/recipes-py