Add E2E backup/restore test for multiple object store credentials

[dsu-igeek]

Describe the problem/challenge you have

Per-Backup Storage Location credentials are being introduced in Velero 1.6. These should be tested

Describe the solution you'd like
Have a test that sets up two backup storage locations with different credentials on AWS, Azure and vSphere.
Ensure that backup/restore can occur successfully with both BSLs
Check with Restic and volume snapshots

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[zubron]

@dsu-igeek Do you think that this can be closed based on the test added in #3559 or would you like to see additional test cases?

The test in that PR allows the creation of another BSL (alongside the default) and can show backup and restore using restic. It does not include volume snapshots though.

[dsu-igeek]

I think we should close this and we'll add new tasks as necessary. Thanks!