Fix type of 'node_whitelist' request parameter

If opportunistic_locking is used, then 'node_whitelist' parameter passed
to the allocator is set to the LU's owned node locks. However, LU owned_locks
has type of 'set' while IReqInstanceAlloc expects type of
'ht.TMaybeListOf(ht.TNonEmptyString)'.

Signed-off-by: Christos Stavrakakis <cstavr@grnet.gr>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>