Add support for gardener provider in clusterloader2

[KesavanKing]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR adds support of Gardener Provider in clusterloader2 framework.

Special notes for your reviewer:

Gardener already takes care of Conformance test coverage - testgrid view. This PR really helps to execute Performance tests for Gardener.

Gardener has its own managed Prometheus. So the prometheus client is created by consuming configurations from Provider Config.