Emit bitwidths of P4 fields

[pudelkoM]

gen-p4-constants.py should emit constants for the bitwidths of P4 fields and headers, so we can remove stuff like this:

fabric-tna/src/main/java/org/stratumproject/fabric/tna/behaviour/FabricInterpreter.java

Line 52 in e87824a

private static final int PORT_BITWIDTH = 9;

[ccascone]

Bitwidth for header fields are not needed since the onos core takes care of reading them from the p4info when translating/building table entries. I added support for emitting bitwidth constants for controller packet metadata, since the onos core doesn't do any translation for packet in/out operations.