Make discover-tempest-conf work with custom certificate

[chanchiwai-ray]

The original issue is in the upstream: when a custom certificate is used and OS_CACERT is set, the discover-tempest-conf still cannot verify the certificate because internally it uses certifi as a CA provider. See https://storyboard.openstack.org/#!/story/2011168.

Once the upstream is fixed, snap-tempest still need to make sure the OS_CACERT path is pointed to a location where the snap is accessible to.