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>

ipaserver/plugins/dogtag.py

@@ -250,6 +250,7 @@ import pki
 from pki.client import PKIConnection
 import pki.crypto as cryptoutil
 from pki.kra import KRAClient
+import six
 
 from ipalib import Backend
 from ipapython.dn import DN
@@ -258,6 +259,9 @@ import ipapython.dogtag
 from ipapython import ipautil
 from ipaserver.install.certs import CertDB
 
+if six.PY3:
+    unicode = str
+
 # These are general status return values used when
 # CMSServlet.outputError() is invoked.
 CMS_SUCCESS      = 0

ipaserver/plugins/join.py

@@ -21,12 +21,17 @@
 Joining an IPA domain
 """
 
+import six
+
 from ipalib import api
 from ipalib import Command, Str
 from ipalib import errors
 from ipalib import _
 from ipaserver.install import installutils
 
+if six.PY3:
+    unicode = str
+
 
 def validate_host(ugettext, cn):
     """