Você deve procurar no /usr/share/xsessions/
. Para mim no Ubuntu 16.04, tive que adicionar manualmente o arquivo awesome.desktop lá.
$ ls /usr/share/xsessions/
awesome.desktop ubuntu.desktop
$ cat /usr/share/xsessions/awesome.desktop
[Desktop Entry]
Encoding=UTF-8
Name=awesome
Comment=Highly configurable framework window manager
TryExec=awesome
Exec=awesome --no-argb
Type=Application