OS environment: add network information

1) Move the hooks environment dict generator inside the object. This
also adds missing values such as network family and uuid.
2) Use the same generator both for the os environment and for the
instance hooks.
3) Update manpage and hooks documentation.

Also update a forgotten docstring about a tuple's content.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>