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>

doc/examples/python-api.py

@@ -37,9 +37,7 @@ api.finalize()
 # Backend.ldap.connect(), otherwise Backend.rpcclient.connect().
 
 if api.env.in_server:
-    api.Backend.ldap2.connect(
-        ccache=api.Backend.krb.default_ccname()
-     )
+    api.Backend.ldap2.connect()
 else:
     api.Backend.rpcclient.connect()