refactor "username" and password to "key" and "secret" to eliminate confusion

```    
def __init__(self, user: str, password: str, group: str = None,
                 auth_method: Union[HTTPBasicAuth, HTTPDigestAuth] = HTTPDigestAuth):
        self.group = group

```
is misleading... 

the term user should be replaced with key, and the term password to secret .


Idea from discussion #153 