krb5 locator: fix IPv6 support
    
    IPv6 addresses are added with surrounding '[' and ']' to the kdcinfo
    file to be able to specify a port number properly. The Kerberos location
    plugin didn't handle those entries properly.
    
    Related to https://pagure.io/SSSD/sssd/issue/941
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>