Use clab "startup_config" parameter to configure lab devices

[ipspace]

After implementing #2991, we could go one step further: containerlab supports startup_config parameter for many network devices. We could (during the node_files creation process) create a file that's a simple merge of all configuration files and use that file as the "clab.startup_config" parameter, completely skipping the node configuration in netlab initial.

I'm pretty sure this wouldn't work for all devices, and depending on how startup_config is implemented, we might not be able to get away with a simple merge. However, do you think this idea is worth trying @jbemmel @ssasso @sdargoeuves @Muddyblack?