Check the "storagePrefix" parameter at the backend creation

[YvosOnTheHub]

Describe the solution you'd like
Trident does not seem to verify the validity of the storagePrefix parameter.
Adding an extra check would avoid errors at PVC creation.

Describe alternatives you've considered
An alternative would be to add caveats in the documentation with regards to this parameter.

Additional context
In my test, here is the parameter I used:
"storagePrefix": "test/nas1_",
Trident did not complain during the backend creation.

However, an ONTAP error was raised when creating a PVC:

failed to provision volume with StorageClass "storage-class-nas": rpc error: code = Unknown desc = encountered error(s) in creating the volume: [Failed to create volume pvc-46a0cded-a847-4b85-b10a-ca7a968e8717 on storage pool aggr1 from backend NAS_Vol-default: error checking for existing volume: API status: failed, Reason: Invalid character '/' in volume name. It can have '_' and alphanumeric characters., Code: 13001], [Failed to create volume pvc-46a0cded-a847-4b85-b10a-ca7a968e8717 on storage pool aggr2 from backend NAS_Vol-default: error checking for existing volume: API status: failed, Reason: Invalid character '/' in volume name. It can have '_' and alphanumeric characters., Code: 13001]

[gnarl]

We are planning to fix this in the Trident 20.07 release.

[gnarl]

Fixed in commit c262e4c.

[travisghansen]

This removed the ability to use - which seemed to be perfectly fine previously...