Update transit constants
Update transit constants

Official documentation (https://www.vaultproject.io/api-docs/secret/transit#generate-data-key) and your own documentation (https://hvac.readthedocs.io/en/stable/usage/secrets_engines/transit.html#generate-data-key) only mentions "plaintext" and "wrapped" as valid value, while "ciphertext" was written here. Moreover, "ciphertext" is not valid and results in the following traceback : 

Traceback (most recent call last):
  File "[...]", line 91, in runcode
    exec(code, self.locals)
  File "<input>", line 4, in <module>
  File "[...]/hvac/api/secrets_engines/transit.py", line 504, in generate_data_key
    json=params,
  File "[...]/hvac/adapters.py", line 107, in post
    return self.request('post', url, **kwargs)
  File "[...]/hvac/adapters.py", line 342, in request
    response = super(JSONAdapter, self).request(*args, **kwargs)
  File "[...]/hvac/adapters.py", line 309, in request
    errors=errors
  File "[...]/hvac/utils.py", line 37, in raise_for_error
    raise exceptions.InvalidRequest(message, errors=errors, method=method, url=url)
hvac.exceptions.InvalidRequest: Invalid path, must be 'plaintext' or 'wrapped', on post https://[...]
ianunruh/hvac,ianunruh/hvac