salt.utils.which(): use default_path if binary not found

To make salt.utils.which() operate well in salt-call when invoked from
cron (where PATH may be present but limited), try the paths in
default_path if the binary is not found in the search_path.
Make salt.utils.network.linux_interfaces work in cron
Merge pull request #9148 from terminalmage/issue7990

Make network.interfaces work in cron where PATH is limited