[CBBS] Improve behaviour around handling provider prefixes

Problem

When setting up a cloud repository on the backup service, the UI prepends the prefix of the given cloud provider (here), regardless of whether the bucket name already has that included. For example, a GCP archive with an entered bucket name "gs://test-bucket" will be sent to the backup service endpoint as "gs://gs://test-bucket". The CBBS code will end up parsing the bucket name as "gs" (here), which will result in the somewhat cryptic error:

Could not add repository: Location not accessible by all nodes - node 80c719baa447d5c2ad82db10cdd0634a cannot access location gs://gs://test-bucket/: could not create node file due to a cloud error: googleapi: Error 400: Invalid bucket name: 'gs:', invalid

Fix

From a UX perspective, the best solution would probably be to detect the provider path ("gs://", "az://" or "s3://") and if the provider selected matches, and either automatically remove it from the bucket name before sending it to the backup service, or return a more informative error under the cloud bucket name box on the UI.