Work in progress: Use asymmetric.utils.Prehashed from cryptography 1.6

[stavxyz]

https://cryptography.io/en/latest/hazmat/primitives/asymmetric/utils/#cryptography.hazmat.primitives.asymmetric.utils.Prehashed

Tests aren't passing yet, still need to dig in.

Previous logic was using stavxyz/cryptography@fe0f048

My first go went like this:

        signature = self.private_key.sign(
            data,
            padding.PKCS1v15(),
            utils.Prehashed(hashes.SHA1())
        )

but we get this:

Traceback (most recent call last):
  File "~/requests-chef/requests_chef/mixlib_auth.py", line 110, in __call__
    signed = self.private_key.sign(canonical_request, b64=True)
  File "~/requests-chef/requests_chef/mixlib_auth.py", line 191, in sign
    utils.Prehashed(hashes.SHA1())
  File "~/.virtualenvs/requests-chef-dev/lib/python2.7/site-packages/cryptography/hazmat/backends/openssl/rsa.py", line 459, in sign
    self._backend, data, algorithm
  File "~/.virtualenvs/requests-chef-dev/lib/python2.7/site-packages/cryptography/hazmat/backends/openssl/utils.py", line 42, in _calculate_digest_and_algorithm
    "The provided data must be the same length as the hash "
ValueError: The provided data must be the same length as the hash algorithm's digest size.

So I changed to:

        signature = self.private_key.sign(
            hashlib.sha1(data).digest(),
            padding.PKCS1v15(),
            utils.Prehashed(hashes.SHA1())
        )

but the tests fail, and I assume requests to a chef server would also fail.