Alias "unicode" to "str" under Python 3

The six way of doing this is to replace all occurences of "unicode"
with "six.text_type". However, "unicode" is non-ambiguous and
(arguably) easier to read. Also, using it makes the patches smaller,
which should help with backporting.

Reviewed-By: Petr Viktorin <pviktori@redhat.com>

ipalib/parameters.py

@@ -123,6 +123,9 @@ from ipapython.dnsutil import DNSName
 def _is_null(value):
     return not value and value != 0 # NOTE: False == 0
 
+if six.PY3:
+    unicode = str
+
 class DefaultFrom(ReadOnly):
     """
     Derive a default value from other supplied values.