Pass proper GCP project with snapshots storage to PrometheusController

[tosi3k]

When using pod-utils for SIG scalability test jobs, clusterloader was attempting to snapshot Prometheus' PDs to Prow cluster project. The proposed flag experimental-snapshot-project (which defaults to PROJECT environment variable value which is set here) is added to be utilized by gcloud command that performs snapshotting.

/sig scalability

[k8s-ci-robot]

Hi @tosi3k. Thanks for your PR.

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[wojtek-t]

/ok-to-test

[wojtek-t]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tosi3k, wojtek-t

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