Fix LDAP object parameter encoding
    
    Parameters in LDAP objects missed an information if they are real
    LDAP attributes or not. Real LDAP attributes are written to
    entry_attrs dictionary in plugin callbacks and are being encoded.
    This causes issues when plugin callbacks does not expect that
    the parameters values are already encoded for submission to LDAP.
    
    This patch introduces a new flag "noattribute" used to mark that
    a parameter is not an LDAP attribute and thus should not be encoded
    or added to entry_attrs. Param documentation is improved to describe
    the meaning of this and other Param flags or attributes.
    
    https://fedorahosted.org/freeipa/ticket/2097