Adding an oxm for mpls BoS indicator bit

The java build error is an artifact of the value declared as ```uint8_t```, which we currently do not handle by default in constructing the OXM class. Also, the value is logically a boolean, so we may want to express it as such in Java.

You can fix this by adding a rule for of_oxm_mpls_bos to java_gen/java_type.py, dictionary exceptions. Take a look at ```of_oxm_bsn_global_vrf_allowed``` for a blueprint.