Install Python 2 on Ubuntu Xenial
Install Python 2 on Ubuntu Xenial

Ubuntu 16.04 (Xenial) only has Python 3 installed by default.
But diskimage-builder currently relies on Python 2 being present,
so we need to install the corresponding package, "python".

Change-Id: I3d31147229f53581f3201c7f053157c3024caa36
Signed-off-by: Simon Leinen <d3959b547a87f34cf375ed11d3673056b93b5378@gmail.com>
Closes-bug: 1577105

switch-ch/diskimage-builder,switch-ch/diskimage-builder