The Splunk Add-on for Google Cloud Platform is using the httplib2 library.  What worked for us was to set the HTTPLIB2_CA_CERTS environment variable in the Splunk systemd unit file and point it to the system CA bundle (in our case /etc/ssl/ca-bundle.pem).  Have a look at 'lib/httplib2/certs.py' to understand the logic and alternative solutions.    ... View more