Missing package dependancies added.

A missing "config" parameter for qemu-nbd based seeding
method added.

A new seeding method utilising Cloud-init added.
The qemu-nbd based method is still a default method
for backward compatibility.

To enable cloud-init, set the "seed" parameter on
a cluster or node level to "cloud-init".
To disable seeding, set this parameter to "false".
Setting this parameter to "true" will default to
the "qemu-nbd" method.

Salt Minion config file will be created automatically
and may be overrided via cluster or node level
metadata:

  salt:
    control:
      cluster:
        mycluster:
          seed: cloud-init
          cloud_init:
            user_data:
              salt_minion:
                conf:
                  master: 10.1.1.1

or for qemu-nbd case:

  salt:
    control:
      cluster:
        mycluster:
          seed: true
          config:
            host: 10.1.1.1

That may be useful when Salt Master has two IPs in
different networks and one of the networks isn't accessible
from a VM at the moment it's created. Setting a reachable
Salt master IP from metadata helps avoid potential problems.

Also, a liitle optimization has been done to parse/dump
an libvirt XML only once while modifying it.

Change-Id: I091cf409cb43ba2d0a18eaf2a08c11e88d0334e2
Closes-Bug: PROD-22191

This patchset obsoletes switching from "dhcp" to "static" during
deployment.

Example pillars:
salt:
  control:
    enabled: true
    virt_enabled: true
    size:
      small:
        cpu: 1
        ram: 1
    cluster:
      infra:
        domain: example.com
        engine: virt
        config:
          engine: salt
          host: master.domain.com
        cloud_init:
          network_data:
            links:
            - id: ens2
              name: ens2
              type: phy
          user_data:
            disable_ec2_metadata: true
            resize_rootfs: True
            timezone: UTC
            ssh_deletekeys: True
            ssh_genkeytypes: ['rsa', 'dsa', 'ecdsa']
            ssh_svcname: ssh
            locale: en_US.UTF-8
            disable_root: true
            apt_preserve_sources_list: false
            apt:
              sources_list: ""
              sources:
                ubuntu.list:
                  source: ${linux:system:repo:ubuntu:source}
                mcp_saltstack.list:
                  source: ${linux:system:repo:mcp_saltstack:source}
        node:
          ctl01:
            provider: kvm01.example.com
            image: ubuntu-16.04.qcow2
            size: small
            cloud_init:
              network_data:
                networks:
                - id: private-ipv4
                  ip_address: 192.168.0.161
                  link: ens2
                  netmask: 255.255.255.0
                  routes:
                  - gateway: 192.168.0.1
                    netmask: 0.0.0.0
                    network: 0.0.0.0
                  type: ipv4

Change-Id: I087518404c61e0bab2303e40957bacca94692102