Teach pylint how our api works
    
    pylint 2.0 is more strict and complains about several aspects of
    ipalib.api. It turns out that AstroidBuilder.string_build() can be used
    to easily teach pylint about object attributes and attribute values.
    Although the assignment wouldn't work with the actual implementation,
    the string builder assignments shows pylint the names and values of
    members. It works without additional transformation.
    
    See: https://pagure.io/freeipa/issue/7614
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Armando Neto <abiagion@redhat.com>