move unpack logic to pyof

[erickvermot]

As we have discussed prior to version b3, pyof doesn't have yet a version independent unpack function, and is not yet capable of unpacking Hello messages with a body properly, nor hello messages of version different from those available in pyof.
So that a proper handshake could be achieved, we have implemented a few utility methods in of_core.utils . The logic in these methods need to be incorporated in pyof, so that a single call to pyof.utils.unpack is necessary, instead of having to separate the hello unpack (GenericHello https://github.com/kytos/kytos-napps/blob/master/napps/kytos/of_core/main.py#L135) from the other unpacks (unpack https://github.com/kytos/kytos-napps/blob/master/napps/kytos/of_core/main.py#L152).

For this, we need first to:

• move the unpack method to pyof. (it will also make sure that tests in pyof behave in the same way).
• move packet validation to inside the unpack method (checks of length and valid version).
• replace the call to of_core.utils.unpack to a call to pyof.utils.unpack
• place tests for multiple hello messages with multiple versions and bodies.
• fix the pyof hello unpack.
• make sure it only raises UnpackException

And once pyof.utils.unpack is capable of unpacking any hello message:

• replace the separate calls to unpack and GenericHello to a single call to pyof.utils.unpack.

make tests use the same unpack method as the lib - kytos/python-openflow#396
Provide a version independent unpack method - kytos/python-openflow#381
Version Unpacking Problems - kytos/python-openflow#344
Hello Messages compliance - optional elements - kytos/python-openflow#379
v0x04 Hello unpack bug - kytos/python-openflow#378