IPA: Parse qualified names when guessing AD user principal
    
    Most AD users store their UPN in an attribute. If they don't, or the sssd
    was configured (typically in earlier versions to work around a bug) to not
    look at the principal attribute, then sssd is supposed to guess
    the attribute.
    
    That currently doesn't work in 1.14, because the username is already
    qualified and then we also append the realm name to it. We need to parse
    the simple username from the qualified name first.
    
    The issue can be reproduced simply by authenticating as the Administrator
    account in IPA-AD trust setups.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3127
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>