UI refers to "Release" instead of "Series"

[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.]

+    <label for="id_distro_series">Release</label>
+    {{ node_form.distro_series }}
...
+    distro_series = forms.ChoiceField(
+        choices=DISTRO_SERIES_CHOICES, required=False,
+        initial=DISTRO_SERIES.default,
+        label="Release",
+        error_messages={'invalid_choice': INVALID_DISTRO_SERIES_MESSAGE})

s/Release/Series/g ?

Also, how about showing the effective series? See bug 1054515.