bionic: DNS search domains lost from cloud-init to netplan

This bug was originally filed in Launchpad as [LP: #1774540](https://bugs.launchpad.net/cloud-init/+bug/1774540)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2018-06-01T00:30:23.851349+00:00

date_fix_committed = None

date_fix_released = None

id = 1774540

importance = medium

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1774540

milestone = None

owner = fo0bar

owner_name = Ryan Finnie

private = False

status = confirmed

submitter = fo0bar

submitter_name = Ryan Finnie

tags = []

duplicates = []

_Launchpad user **Ryan Finnie(fo0bar)** wrote on 2018-06-01T00:30:23.851349+00:00_

On a deployed node with a MAAS datasource, /etc/cloud/cloud.cfg.d/50-curtin-networking.cfg contains:

network:
  config:
  - id: enp1s0
    mac_address: 52:54:00:9a:b4:64
    mtu: 1500
    name: enp1s0
    subnets:
    - address: 10.48.7.48/21
      dns_nameservers:
      - 10.48.0.5
      gateway: 10.48.0.1
      type: static
    type: physical
  - address:
    - 10.48.0.5
    search:
    - bos01.canonistack.internal
    type: nameserver
  version: 1

But the generated /etc/netplan/50-cloud-init.yaml contains no search domains under nameservers for the interface:
# This file is generated from information provided by
# the datasource. Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    version: 2
    ethernets:
        enp1s0:
            addresses:
            - 10.48.7.48/21
            gateway4: 10.48.0.1
            match:
                macaddress: 52:54:00:9a:b4:64
            mtu: 1500
            nameservers:
                addresses:
                - 10.48.0.5
            set-name: enp1s0

Related bugs:
* bug 1750884: /etc/resolv.conf not configured correctly in Bionic, leads to no DNS resolution