Use configurable bucket name for GCS samples data in systems tests. (#8783)
Use configurable bucket name for GCS samples data in systems tests. (#8783)

* Use configurable bucket name for GCS samples data in systems tests.

This allows the VPC-SC team to use their private mirror which is within
the security boundary when testing BigQuery VPC-SC support.

* Blacken

tseaver/google-cloud-python,tswast/google-cloud-python,GoogleCloudPlatform/gcloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,tseaver/google-cloud-python,googleapis/google-cloud-python,googleapis/google-cloud-python,tswast/google-cloud-python,GoogleCloudPlatform/gcloud-python