install-chef-suse: Use network config from /etc/crowbar/network.json

If /etc/crowbar/network.json exists, then we want to use this as the
network configuration (with deep merge).
install-chef-suse: Also call bc-network-admin-helper.rb on network.json

When we have network.json in /etc/crowbar, we also need to validate it.
Merge pull request #1876 from vuntz/install-chef-suse-network

install-chef-suse: Use network config from /etc/crowbar/network.json