gluon-mesh-vpn-fastd: use delete_all instead of foreach to delete the old peers
config-mode, mesh-vpn: refactor key generation

Previously, the config-mode was responsible for generating the fastd
secret. This patch sets the default secret to "generate" causing a
secret to be generated on its first use (e.g. show_key or start).

This also changes the info page (in Expert Mode) to show "n/a" when the
public key is not yet available.
gluon-core: restructure WAN configuration
gluon-luci-portconfig: expose advanced WAN configuration