Fix and rename get_my_domain_data()
    
    The task of get_my_domain_data() is to read some information about the
    configured domain from the cache. While the sysdb interface was
    redesigned some changes changed the behaviour so that the data of the
    domain of the current request was read. If this domain is a sub-domain
    the wrong data was read. As a result group-memberships of the configured
    domain were not taken into account.
    
    The original code didn't made it easy to see that always the parent
    domain should be used here, because there was no comment indication this
    and the function name get_my_domain_data() didn't made it clear either.
    Additionally to fixing the issue this patch also adds a comment and
    rename the function to get_parent_domain_data().
    
    Fixes https://fedorahosted.org/sssd/ticket/1888