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>

install/oddjob/com.redhat.idm.trust-fetch-domains

@@ -10,8 +10,13 @@ from ipalib.constants import DEFAULT_CONFIG
 from ipapython.ipautil import kinit_keytab
 import sys
 import os, pwd
+
+import six
 import gssapi
 
+if six.PY3:
+    unicode = str
+
 def retrieve_keytab(api, ccache_name, oneway_keytab_name, oneway_principal):
     getkeytab_args = ["/usr/sbin/ipa-getkeytab",
                       "-s", api.env.host,