Unclear how to get the effective distro series for a node

[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.]

In src/maasserver/api.py:

+    if node is None or node.status == NODE_STATUS.COMMISSIONING:
+        series = Config.objects.get_config('commissioning_distro_series')
+    else:
+        series = node.get_distro_series()

In src/maasserver/models/node.py:

+    def get_distro_series(self):
+        """Return the distro series to install that node."""
+        if not self.distro_series or self.distro_series == DISTRO_SERIES.default:
+            return Config.objects.get_config('default_distro_series')
+        else:
+            return self.distro_series

There's two levels of overriding here: get_distro_series() can return
something other than node.distro_series, and then the code in the API
view can return something else again.

I suggest putting the logic that's in the API view into
get_distro_series, and renaming the method:

    def get_effective_distro_series(self):
        """Return the distro series to install that node."""
        if node is None or node.status == NODE_STATUS.COMMISSIONING:
            return Config.objects.get_config('commissioning_distro_series')
        elif not self.distro_series or self.distro_series == DISTRO_SERIES.default:
            return Config.objects.get_config('default_distro_series')
        else:
            return self.distro_series