nuageinit: Allow the use of network parameters from network-config

To better comply with the cloud-init specification, we need to
support the configuration of network-related parameters from
the network-config file, which is common in most deployments.

Reviewed by: bapt@
Approved by: bapt@
Differential Revision: https://reviews.freebsd.org/D52419

(cherry picked from commit 95230b248f6412c2d1c416c1e9795c3192cdf750)