nicira: fix and clean ofp_flow_mod_table_id

Fix error raised when calling ofp_flow_mod_table_id.pack() due to
_command property accessor returning a string rather than int expected
by struct.pack.

Instead of fixing property accessor, I removed the _command property in
order to inherit pack and unpack directly from superclass. Both
functions are wrapped by decroators to ensure correct serialization and
unserialization of command field.