Allow to disable SSL validation in gabbi tests

Gabbi does not allow to pass custom path to (self-signed) SSL certs,
so we can't really re-use the CONF.identity option (as does the AODH
test client).

This patch adds a new config option

  CONF.telemetry.disable_ssl_certificate_validation

(False by default) that will disable SSL cert validation when running
gabbits.

For comparison, see similar patch to heat's tempest plugin
https://review.opendev.org/#/c/703335

Change-Id: I298c8ebb45f0768b12919cf4f67462df0ff4c631
2 files changed