[Feature] Allow vtbackup pod spec to be override-able.

[bluecrabs007]

DESCRIPTION

Expose the vtbackup pod spec via the VitessCluster object spec. This way, we can change the vtbackup pod spec to be different from vttablet pod spec, which it currently inherits the spec from.

USE CASE

We rely on the vtbackup's feature of --initial_backup to create an empty backup during shard creation. Since the spec is inherited from vttablet, PVCs are set and disks are allocated to these pods. In large clusters, this often leads to additional capacity allocation that sometimes is unavailable.
Ideally we change the spec and remove the PVC allocation or change the storageclass to different disks.