Always update cached upn if enterprise principals are used
    
    Instead of continuing to use the initial upn if enterprise principals
    are used if should always be replaced. The enterprise principal
    is stored in the credential cache and without knowing it the
    ccache_for_princ() calls to determine the location of the credential
    cache will fail.
    
    Fixes https://fedorahosted.org/sssd/ticket/1921