Try DISTKey migration only if key is specified

[alok87]

Otherwise it can be AUTO or manually maintained by the user

Separate note: #211 DIST key support needs to be added for multi node setups.

After this change, loader will only log the diff:

I0502 10:29:01.294592   12633 redshift.go:1089] customers, SortKey is different, SortKey in config: [], target: [id]
I0502 10:29:01.294602   12633 redshift.go:1096] customers, SortKey is AUTO or manually modified, skipped.
I0502 10:29:01.294610   12633 redshift.go:1115] customers, DistKey is different, DistKey in config: [], target: [user_id]
I0502 10:29:01.294617   12633 redshift.go:1122] customers, DistKey is AUTO or manually modified, skipped.