python/ovs/db/types: Fix English grammar for enums with one member.

Before this change, enums that have one member were formatted as, e.g.:
    "one of xyzzy, , or "
This changes them to be formatted as:
    "must be xyzzy"
which makes much more sense.

(An enum with one member may make some sense if you are trying to leave
the possibility for future expansion.)

Signed-off-by: Ben Pfaff <blp@nicira.com>

python/ovs/db/types.py

@@ -287,7 +287,9 @@ class BaseType(object):
         if self.enum:
             literals = [value.toEnglish(escapeLiteral)
                         for value in self.enum.values]
-            if len(literals) == 2:
+            if len(literals) == 1:
+                english = 'must be %s' % (literals[0])
+            elif len(literals) == 2:
                 english = 'either %s or %s' % (literals[0], literals[1])
             else:
                 english = 'one of %s, %s, or %s' % (literals[0],