Force DIB Python version for py2 in diskimage-create
Force DIB Python version for py2 in diskimage-create

DIB automatically picks a Python version based on the distro and
version. For Ubuntu Bionic, that default is Python 3 which means the
amphora-agent does not run on Python 2 when it was expected to on -py2
jobs [1].

[1] http://logs.openstack.org/10/661310/8/check/ \
    octavia-v2-dsvm-py2-scenario/664a91b/controller/logs/dib-build/ \
    amphora-x64-haproxy.qcow2_log.txt.gz#_2019-07-27_15_51_58_393

Change-Id: I6fd12be06bc848b3e29828c091f61229703e3a57

openstack/octavia,openstack/octavia,openstack/octavia