Amphora images based on RHEL flavors should use Python 3 interpreter same as in Ubuntu

[Nir Magnezi]

All amphora images are expected to run[1] the amphora agent using the same Python version.
What we currently have is as follows:
Ubuntu: Python 3.5.2 (expected)
RHEL, CentOS, Fedora: Python.7.5

The reason is:
https://github.com/openstack/diskimage-builder/blob/master/diskimage_builder/elements/dib-python/environment.d/50-dib-python-version
https://github.com/openstack/diskimage-builder/blob/master/diskimage_builder/elements/dib-python/pre-install.d/01-dib-python

We should change (probably override in Octavia code base) the image building for RHEL, CentOS and fedora to use Python 3 to avoid future bugs in the amphora agent.
This is already breaking this[1] WIP patch Which only works on Ubuntu based amphora since it has some Python3 specific code bits:

[1] via systemd: /usr/local/bin/dib-python /usr/local/bin/amphora-agent --config-file /etc/octavia/amphora-agent.conf
[1] https://review.openstack.org/#/c/429398
[2] https://review.openstack.org/#/c/429398/31/octavia/amphorae/backends/agent/api_server/listener.py@124

[Bernard Cafarelli (bcafarel)]

If I got it correctly, this is how we end with python2:
* DIB setups dib-python element as a dependency on pip-and-virtualenv-source-install element
* this element installs/reinstalls pip and virtualenv with forced /usr/local/bin/dib-python
* new pip now is "configured" for /usr/local/bin/dib-python
* pip install in /opt/amphora creates the amphora-agent console_script entry point with this interpreter
* systemd unit just uses this script

Not sure where to mix in a change to force python 3 yet (also it will most probably need a step to install proper python3)

[Nir Magnezi (nmagnezi)]

Tested a CentOS based amp again (with current master branch):

[root@amphora-cc38f42f-8875-4813-82f9-35c8c1c7354e ~]# ps -ef | grep amp
root 3277 742 0 09:34 ? 00:00:00 /usr/bin/python /usr/local/bin/amphora-agent --config-file /etc/octavia/amphora-agent.conf
root 9335 9321 0 09:35 pts/0 00:00:00 grep --color=auto amp
[root@amphora-cc38f42f-8875-4813-82f9-35c8c1c7354e ~]# /usr/bin/python --version
Python 2.7.5

[Gregory Thiemonge (gthiemonge)]

Abandoned after re-enabling the Octavia launchpad.