Change default DB path to /etc/fusa-gcc-plugin/valid_apis.json

Currently the plugin is looking for json DB file as valid_apis.json in the current directory.

We should use a stable location for that.

Expected behavior:

• If the parameter -fplugin-arg-fusa-database-file is specified - the plugin will look for the DB file in the specified location, and will fail if not found.
• Else, the plugin will look for /etc/fusa-gcc-plugin/valid_apis.json, and will fail if not found