Support for manual mode of interfaces

This bug was originally filed in Launchpad as [LP: #1736915](https://bugs.launchpad.net/cloud-init/+bug/1736915)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2017-12-07T09:52:52.954801+00:00

date_fix_committed = None

date_fix_released = None

id = 1736915

importance = medium

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1736915

milestone = None

owner = hudayou

owner_name = Hengqing Hu

private = False

status = triaged

submitter = hudayou

submitter_name = Hengqing Hu

tags = []

duplicates = []

_Launchpad user **Hengqing Hu(hudayou)** wrote on 2017-12-07T09:52:52.954801+00:00_

https://review.openstack.org/#/c/520590/

Need to configure the first network interface like this, but didn't find a way to set it manual and also auto guess the name of the first interface unless special cloud image is made.

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual

auto eth0
iface eth0 inet6 auto

source /etc/network/interfaces.d/*