Support for trusted client with Grid’5000

Internal certificate. In this mode users can pass the g5k_user argument
to most calls to specify which user the API call should be made as. In
cases where g5k_user is not specified API calls will be made as the
anonymous user whose access is limited to the Grid’5000 reference API.
In this mode python-grid5000 does not store any login information, so
g5k_user must be provided explicitly provided on every call that requires
one.