Add type=cloud-init for --static-iface option

When image is built without --static-iface option and static address is set up by cloud-init an interface can have duplicate stanzas in both /etc/network/interfaces and in /etc/network/interfaces.d/50-cloud-init. In this case dhclient is launched for itnerface with static address configured by cloud-init.

This commit introduces --static-iface type=cloud-init option which just writes loopback config and source /etc/network/interfaces.d/* in /etc/network/interfaces