Clean up handling of repository-s3/gcs/azure for 8.0+

[b-deam]

Since 8.0, we've converted the repository-azure, repository-gcs and repository-s3 plugins into Elasticsearch modules, so that they are always included. Whilst adding or removing these plugins still succeeds, it is now a no-op.

We should clean up our handling of these plugins for versions 8.0+.

This will require changes to the corresponding rally-teams repo, and then subsequently our current handling of post install hooks to configure the ES keystore, likely by making repository configuration a first class citizen of Rally:

rally/esrally/mechanic/provisioner.py

Line 197 in 159ae64

for installer in self.plugin_installers: