node-proxy: explicitly set NodeProxy's attributes

node-proxy: explicitly set NodeProxy's attributes

The current logic using `setattr()` makes mypy complain:

"NodeProxy" has no attribute "xxx"

Using `self.__dict['xxx']` addresses this mypy error but the
downside of this is that the code isn't clear and less readable.

Explicitly setting the different attributes makes the code clearer
and more readable.

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>
(cherry picked from commit e71bf838428c297075df5515c342c0db0a9e31e3)