Port from python-krbV to python-gssapi

python-krbV library is deprecated and doesn't work with python 3. Replacing all
it's usages with python-gssapi.

- Removed Backend.krb and KRB5_CCache classes
  They were wrappers around krbV classes that cannot really work without them
- Added few utility functions for querying GSSAPI credentials
  in krb_utils module. They provide replacements for KRB5_CCache.
- Merged two kinit_keytab functions
- Changed ldap plugin connection defaults to match ipaldap
- Unified getting default realm
  Using api.env.realm instead of krbV call

Reviewed-By: Jan Cholasta <jcholast@redhat.com>
Reviewed-By: Robbie Harwood <rharwood@redhat.com>
Reviewed-By: Simo Sorce <ssorce@redhat.com>

ipaserver/plugins/join.py

@@ -21,8 +21,6 @@
 Joining an IPA domain
 """
 
-import krbV
-
 from ipalib import api
 from ipalib import Command, Str
 from ipalib import errors
@@ -30,15 +28,6 @@ from ipalib import _
 from ipaserver.install import installutils
 
 
-def get_realm():
-    """
-    Returns the default kerberos realm configured for this server.
-    """
-    krbctx = krbV.default_context()
-
-    return unicode(krbctx.default_realm)
-
-
 def validate_host(ugettext, cn):
     """
     Require at least one dot in the hostname (to support localhost.localdomain)
@@ -66,7 +55,7 @@ class join(Command):
     takes_options = (
         Str('realm',
             doc=_("The IPA realm"),
-            default_from=lambda: get_realm(),
+            default_from=lambda: api.env.realm,
             autofill=True,
         ),
         Str('nshardwareplatform?',