Consider moving CREATE_DISK validations to SLRP.

The CreateDisk.target_type and CreateDisk.target_profile (introduced in MESOS-9275) fields are currently validated by the master. We should consider deferring the validation to SLRP for the following reasons:
1. The validations are very specific to storage resource providers, and we might want to relax that in the future. The code would be more cohesive if all the validations are done at the same place.
2. Specifically, the validation of target_profile is not complete: the master can only validate its existence, but not its validity since the master does not have access to the disk profile adaptor.