checks: Use list of release types from productmd

Let's not duplicate the list. Productmd exports a list of valid values,
so Pungi should just pick and use that.

Closes: https://pagure.io/pungi/pull-request/773
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

doc/configuration.rst

@@ -62,8 +62,10 @@ Options
 **release_version** [mandatory]
     (*str*) -- release version
 
-**release_type** = "ga"
-    (*str*) -- release type, "ga" or "updates"
+**release_type** = "ga" (*str*) -- release type, for example ``ga``,
+    ``updates`` or ``updates-testing``. See `list of all valid values
+    <http://productmd.readthedocs.io/en/latest/common.html#productmd.common.RELEASE_TYPES>`_
+    in productmd documentation.
 
 **release_is_layered** = False
     (*bool*) -- typically False for an operating system, True otherwise