fixing BZ1820297

[jackyalbo]

Will now verify that storageclass exist, and that it's not an obc only type:

$ nb backingstore create pv-pool pool1 --num-volumes 3 --pv-size-gb 16 --storage-class jacky.noobaa
INFO[0000] ✅ Exists: NooBaa "noobaa"
INFO[0000] ❌ Not Found: StorageClass "jacky.noobaa"
FATA[0000] ❌ Could not get StorageClass "jacky.noobaa" for system in namespace "jacky"
$ nb backingstore create pv-pool pool1 --num-volumes 3 --pv-size-gb 16 --storage-class jacky.noobaa.io
INFO[0000] ✅ Exists: NooBaa "noobaa"
INFO[0000] ✅ Exists: StorageClass "jacky.noobaa.io"
FATA[0000] ❌ Could not set StorageClass "jacky.noobaa.io" for system in namespace "jacky" - as this class reserved for obc only

https://bugzilla.redhat.com/show_bug.cgi?id=1820297