'Ensure the username and password is valid.
OBS! The validation should also set the user attribute of the request
to a valid resource owner, i.e. request.user = username or similar. If
not set you will be unable to associate a token with a user in the
persistance method used (commonly, save_bearer_token).
:param username: Unicode username
:param password: Unicode password
:param client: Client object set by you, see authenticate_client.
:param request: The HTTP Request (oauthlib.common.Request)
:rtype: True or False
Method is used by:
- Resource Owner Password Credentials Grant'

def validate_user(self, username, password, client, request, *args, **kwargs):

    raise NotImplementedError(u'Subclasses    must    implement    this    method.')
