Use Tempest-lib's compute certificates_client

Compute certificates_client has been migrated to tempest-lib-
I225569ddb2c56200cf071e7abd017ce183fd161f

Now Tempest-lib provides this client as stable interface so Tempest
should start using this from lib and remove its own copy.
Also remove other files for this client like response schema, unit tests.

This commits makes Tempest to use compute certificates_client
from Tempest-lib.

Change-Id: Iab4ebe329e721274d98d09bcfed2ac37879013cb
5 files changed