ovs-vsctl: Allow modifying "immutable" columns if we just created the row.

OVSDB has the concept of "immutable" columns, which are columns whose
values are fixed once a row is inserted.  Until now, ovs-vsctl has not
allowed these columns to be modified at all.  However, this is a little too
strict, because these columns can be set to any value at the time that the
row is inserted.  This commit relaxes the ovs-vsctl requirement, then, to
allow an immutable column's value to be modified if its row has been
inserted within this transaction.

Requested-by: Mukesh Hira <mhira@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>