make INCLUDE variable configurable in client PXE config

currently we hardcode:

``` python
    CFG_TEMPLATE = Stringtemplate(dedent('''\
        INCLUDE pxelinux.cfg/default
        DEFAULT instantboot
        PROMPT 0
        TIMEOUT 1
        LABEL instantboot
            KERNEL cmd.c32
            APPEND ${label} ${options}
        '''))
```

we need an option in our marmoset.conf to set the include path to something else.
