[FakeGCS] Add a new field emulatorEnabled in gcp secret & configure Gcloud client for GCS Emulator

[anveshreddy18]

What this PR does / why we need it:

• Add a new field emulatorEnabled in gcp secret
• configure Google cloud client for GCS Emulator when emulatorEnabled is true

This is helpful to run etcdbr tests locally and on pipelines without actually using the real infra. This can also be used by Etcd-druid to use GCS Emulator fake-gcs-server for running e2e-tests.

For these, Google cloud client in backup-restore has to be configured to connect to the Emulator.

This PR enables that in either of the two ways

1. GOOGLE_EMULATOR_ENABLED env var is set when running etcdbr as a process using etcdbrctl
2. emulatorEnabled field in the gcp secret is set to true via helm chart

Which issue(s) this PR fixes:
Fixes #696

Special notes for your reviewer:

Release note:

Added support for using fake-gcs-server for all etcdbr functionalities. To enable: Either 
1) Set `GOOGLE_EMULATOR_ENABLED` environment variable when running `etcdbrctl` command OR 
2) Set `emulatorEnabled: true` in GCP backup secret when deploying via Helm chart.

[shreyas-s-rao]

@anveshreddy18 please also modify the release note to talk about how to enable emulator for etcdbrctl command, rather than from the provider secret perspective. Something like

Added support for using (fake-gcs-server)[https://github.com/fsouza/fake-gcs-server] for taking backups. Please set environment variable `GOOGLE_EMULATOR_ENABLED` when running `etcdbrctl` command to enable this support. Alternatively, you may also set the field `emulatorEnabled: true` in the GCP backup secret when deploying etcd-backup-restore via the helm chart to enable this.

This mentions to users of both etcdbrctl process as well as the helm charts, as to how to enable this feature.

[gardener-robot]

@anveshreddy18 You need rebase this pull request with latest master branch. Please check.

[seshachalam-yv]

/lgtm

[renormalize]

Thanks for also reflecting the changes in Azure with regards to naming convention, @anveshreddy18.
Testing will be significantly easier for GCS and Azure; now that emulators are supported for the three major providers.

Remove the unnecessary commit messages from the squash commit before merging for a cleaner commit message and history.

Thanks!