Node.distro_series and related changes are not fully tested

[Gavin Panella]

[lp:~andreserl/maas/distro_series_support was merged moments before my
review landed; this bug documents one of the points that still needs
addressing.]

Missing tests:

- That KernelParameters returned from the pxeconfig view includes the
  *effective* series of a node.

- Exercise the updated form field in src/maasserver/forms.py.

- Tests for get_effective_distro_series() - see bug 1054515 - when:

  - node is None,

  - status is COMMISSIONING,

  - self.distro_series is None or "",

  - self.distro_series is default,

  - otherwise.

- Browser test for the change in
  src/maasserver/templates/maasserver/snippets.html.