Do not switch to credentials everytime.
    
    If user decide to kinit as another user we do not want to switch back
    to user ccache at another login. We will switch to new ccache if and only
    if default principal name is the same as current principal name, or there is
    not any default ccache.
    
    https://fedorahosted.org/sssd/ticket/1936
    
    Reviewed-by: Pavel Reichl <preichl@redhat.com>