setup.py: need curl on Ubuntu

We need "curl" for our default "make" (it is used to fetch Lua for the
default image's CLI). So add this requirement for Ubuntu in setup.py.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>