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.

_command property has been replaced with decorator `splice_table_id`
which temporarily injects `table_id` into MSB of `command` during
wrapped function call.

Closes #81