Core: Initial network layout, management and firewall setup

[travier-anssi]

• [X] Initial network layout using network namespaces:
  • [X] ipsec0 network namespace with exclusive access to IPsec network
  • [X] XFRM interface for IPsec support
• [X] Initial static nftables-based firewall setup for all network namespaces:
  • [X] Strict network access (input & output) controls
  • [X] IPsec awareness
• [X] IPv4 only support
• [X] Testbed infrastructure update