Multi-threaded script-based device configuration

[ipspace]

"netlab initial" uses namespace bash scripts to configure Linux nodes, and container-side bash scripts to configure some daemons. After #2991, we could use bash scripts to configure FRR nodes.

While I'm pretty sure this is already much faster than using Ansible (I know I posted a few performance figures in one of the issues or PRs, but can't find them anymore), imagine adding multi-threading ;)