pynag doesn't list services applied via a hostgroup when multiple levels of appends are used in the config

When a host is defined via multiple levels of inheritance containing hostgroups being appended at each level, pynag get_effective_hostgroups doesn't properly find all of the associated hostgroups with the leaf host.

The main issue is that in my situation, pynag does not return "mytestgroup" via get_effective_hostgroups, which means the "My Test Service" check_command can't be returned for the host "my-server".  I'm not sure what is wrong with ObjectRelations.host_hostgroups, but I noticed _get_effective_attribute exists as some kind of helper function to deal with this use case, yet it is never used anywhere.  A similar fix could be applied to contactgroups, servicegroups, etc.

This problem was seen in a large complex environment, but I haven't reproduced it yet in a simplified Nagios install, so stay tuned.  

Below is a representation of the config.
define hostgroup {
    hostgroup_name    mytestgroup
}
define hostgroup {
    hostgroup_name    my-web-node
}

define host {
    name                     my-server
    use                        linux-server
    hostgroups            +myproject,mytestgroup
    register                  0
}
define host {
    name                    my-server-development
    use                       my-server
}
define host {
    use                     my-server-development
    host_name         myhost
    hostgroups         +my-web-node
    address              127.0.0.1
}
define service{
    service_description My Test Service
    use                    local-service
    check_command dummy_command
    hostgroup_name mytestgroup
}
define command{
    command_name  dummy_command
    command_line     $USER1$/check_dummy 0 "My dummy check"
}