make consistent GenericType initialization

[erickvermot]

GenericType accepts a value named argument in its initialization that defaults to None. I propose we keep the GenericType derivatives consistent, naming their initialization argument value and letting them accept None.

BinaryData case is treated in: https://github.com/kytos/python-openflow/issues/420

and Pad is treated as a special case.

The proposed modifications were introduced in #392 , and is now isolated in another branch (https://github.com/kytos/python-openflow/compare/master...erickvermot:generictype_init_value_pr) since #392 was rejected.