[openstacksdk] Create image doesn't validate checksum correctly using sha256 algorithm

I have set config option ``hashing_algorithm`` value as sha256 in glance.

Now, I'm trying to create an image using openstacksdk.

I have set hash value of the image to sha256 parameter of create_image method but it fails with an error "Image checksum verification failed".

Reason: glance store calculates checksum using md5 algorithm and it calculates owner_specified.openstack.sha256/os_hash_value of an image using the algorithm that's set in ``hashing_algorithm``. In openstacksdk, it compares the checksum as shown below:

  checksum = data.get('checksum')
  if checksum:
      valid = (checksum == md5 or checksum == sha256)
      if not valid:
          raise Exception('Image checksum verification failed')

IMO, except md5 algorithm, it should compare sha256 with the os_hash_value that's calculated and set by glance for an image.

for cirros-0.4.0-x86_64-disk.img image:-
md5 checksum is 443b7623e27ecf03dc9e01ee93f67afe
sha256 checksum is a8dd75ecffd4cdd96072d60c2237b448e0c8b2bc94d57f10fdbc8c481d9005b8

If I pass sha256 parameter to create_image as a8dd75ecffd4cdd96072d60c2237b448e0c8b2bc94d57f10fdbc8c481d9005b8, it fails to create an image.