[FIX] tools: find_in_path: config is not ready at import time

commit f76d4525 was not actually working: extra keys from
config files are not yet into the config options dict at
import time. The fix is to move the logic inside the method,
like in `find_pg_tool` just below.

Also fix the use of `find_in_path` in report.py: the subprocess
may also raise AttributeError exception, so instead of listing
all the possible ones just re-raise the IOError shallowed by
`find_in_path` when the result is None.

Fixes #3809 #3811