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>

Reviewed-By: Petr Vobornik <pvoborni@redhat.com>

Some parts of install scripts used only ccache name as returned by
krbV.CCache.name attribute. However, when this name is used again
to initialize krbV.CCache object or when it is used in KRB5CCNAME
environmental variable, it fails for new DIR type of CCACHE.

We should always use both CCACHE type and name when referring to
them to avoid these crashes. ldap2 backend was also updated to
accept directly krbV.CCache object which contains everything we need
to authenticate with ccache.

https://fedorahosted.org/freeipa/ticket/3381

The changes include:
 * Change license blobs in source files to mention GPLv3+ not GPLv2 only
 * Add GPLv3+ license text
 * Package COPYING not LICENSE as the license blobs (even the old ones)
   mention COPYING specifically, it is also more common, I think

 https://fedorahosted.org/freeipa/ticket/239