Configure enabled and disabled extensions in tempest

This patch adds support for configuring the api_extensions
option of the various feature-$service-(enabled|disabled)
sections in tempest's configuration file.

Please note that enabled extensions for the object storage
service are specified in tempest using the discoverable_apis
configuration options.

Partially-implements: Blueprint branchless-tempest-extensions

Change-Id: Ib64c0a474b153f42f3a4a427d86275c85f107997
Merge "Configure enabled and disabled extensions in tempest"