schema plugin: Generate stable fingerprint

If some Param defines several values for `exclude` or `include`
attributes then API schema hash will be unstable.

First, these Param's attributes are converted to frozenset
(ipalib/parameters.py), then `ipaserver.plugins.schema` plugin
converts `exclude` and `include` attrs to list. Set/frozenset in
turn, is unordered collection [0]. So, the end order of values is
undefined.
But due to the nature of sets:
> two sets are equal if and only if every element of each set is
contained in the other (each is a subset of the other)

the order of values can be ignored.

Note: other Param's attrs with type frozenset are not affected because
they are not processed by the schema plugin.

[0]: https://docs.python.org/3/library/stdtypes.html#set-types-set-frozenset

Fixes: https://pagure.io/freeipa/issue/8955
Signed-off-by: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>

ipaserver/plugins/schema.py

@@ -592,7 +592,7 @@ class param(BaseParam):
                 obj[key] = unicode(value)
             elif key in ('exclude',
                          'include'):
-                obj[key] = list(unicode(v) for v in value)
+                obj[key] = sorted(list(unicode(v) for v in value))
             if isinstance(metaobj, Command):
                 if key == 'alwaysask':
                     obj.setdefault(key, value)