curtin:xenial:amd64 Install custom kernel

  * MAAS does not give any possibility to install
    custom kernel version, via node definition
  * Add possilibility to define:
    - exact kernel version
    - few extra pkgs, to be installed during
      curtin stage
  Warning: in that case, global maas variable
    'maas_config:default_min|max_*_kernel:' will
    be ignored at all.

Change-Id: I89f4f656c5e02e1cb10d6a81905b4abd7d738d40
Closes-Bug: PROD-25084 (PROD:25084)
3 files changed