Only use dhcp if enable_dhcp is set on the network

Also removes an unused call to init_host if there is no network
to setup. The signature of this call was wrong so clearly it was
not being used.

Partially-implements blueprint better-support-for-multiple-networks

Change-Id: I33c86b1500f9a69081b4c6ecbe79151f10f69cea
Merge "Only use dhcp if enable_dhcp is set on the network"