env_process: Fix behavior of convert_ppm_files_to_png param

Due to a mistake of the feature authors, the param
convert_ppm_files_to_png is not set in base.cfg. Let's
fix this mistake, and set the behavior more explicitly.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1283 from autotest/fix-preprocessing-param

env_process: Fix behavior of convert_ppm_files_to_png param