Automatic IP address attribution

[louisroyer]

• Add subnet cidr in config
• generate and memorize ip addr on first call of ipv4 or ipv6 (unless specific ip in config file)

• Create assign_ipv4 / assign_ipv6:
• if ip is assigned multiple time, raise an error
• if at exit of python script ipv4/ipv6 has been used but there is no ip assigned (either in template or in config), raise an error