pki-base: use pki-base-python2 as dependency
    
    pki-base provides pki-base-python2, but we should depend directly on
    pki-base-python2 because in future pki-base may provide pki-base-python3
    instead.
    
    Source: cheimes@redhat.com
    
    https://fedorahosted.org/freeipa/ticket/4985
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>