Switch network dep filter to test for ifupdown/nplan instead of release name
Curtin will install some additional packages in-target depending on the
configuration. For example, if the curtin config includes bridges,
then it will ensure that bridge-utils is installed. These deps are
valid for releases using ifupdown. In Artful and newer (Bionic) netplan
is used instead and these tools are not needed.
This patch changes a release based check to a package install check.
If a system does not have ifupdown and also has netplan (nplan package)
then it will NOT install any ifupdown related deps (bridge-utils, vlan,
ifenslave).