gnt-network: Fix import for textwrap

The style guide says to use “import foo”-style imports, not “from foo
import bar” unless it's a Ganeti module. There are some places with
exceptions, but this one certainly isn't warranted. Also fix the import
order.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>