Configure network overlays from config file during boot

[dn0]

Affects the net-physical script (https://github.com/erigones/esdc-erigonos-overlay)

(Moved from https://github.com/erigones/smartos-live/issues/1)

[dn0]

Overlay definitions in the config file:

overlay_rule_<rule_name>="<definition>"

Examples:

overlay_rule_myoverlay="-e vxlan -s direct -p vxlan/listen_ip=2.3.4.5 -p direct/dest_ip=1.2.3.4 -p direct/dest_port=4789 -p mtu=1400"

overlay_rule_second1="-e vxlan -p vxlan/listen_ip=0.0.0.0,vxlan/listen_port=4790 -s files -p files/config=/opt/custom/overlays/second1_overlay.json -p mtu=1400"

From these config lines, the /var/run/smartdc/networking/overlay_rules.json will be created by network/physical (or possibly a new SMF networking script) in format:

{
    "overlay_myoverlay": "-e vxlan -s direct -p vxlan/listen_ip=2.3.4.5 -p direct/dest_ip=1.2.3.4 -p direct/dest_port=4789 -p mtu=1400",
    "overlay_second1": "-e vxlan -p vxlan/listen_ip=0.0.0.0,vxlan/listen_port=4790 -s files -p files/config=/opt/custom/overlays/second1_overlay.json -p mtu=1400"
}

After implementing these changes, the VMs with the overlay networks configured will be able to start right after boot.