python-cheetah: new package

[Thomas:
 - Remove dependency on host-python-markdown, apparently not needed
   (it builds fine in a minimal chroot, which doesn't have
   python-markdown installed)
 - Remove runtime dependency on markdown, I can run Cheetah basic
   examples without python-markdown installed.
 - Add dependency on Python 2 only, since it doesn't build for Python
   3.]

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>