make consistent GenericType initialization

[ajoaoff]

Original issue opened by @erickvermot at kytos#426.

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: kytos#420

and Pad is treated as a special case.

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