krb5: Use new function to validate ccaches
    
    This function replaces and combines check_for_valid_tgt() and type specific
    functions that checked for ccache existence by using generic krb5 cache
    function and executing them as the target user (implicitly validate the
    target use rcan properly access the ccache).
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2061