Open Stack

Hello Everyone,

Devstack has been default to python3 since ussuri release and Victoria cycle is time to drop the
support of python2 completely.

- https://review.opendev.org/#/c/737455/

Being ussuri as python3-only release and all grenade jobs has been migrated to python3 in ussuri, we
are all good here but in case your project is still using the py2 jobs or replying on python2 support in devstack
in any extend, do ping us on #openstack-qa channel. 

We will be keeping USE_PYTHON3 variable support till all consumer remove this, like storlets to adjust the requirement.

NOTE: branchless repo keeping common jobs for master and stable branch gate with USE_PYTHON3=False need to
keep setting this var for configuring the devstack with right python. For example, tempest-full job definition in tempest master.
Such jobs should be run till stable/train only as ussuri onward we tests things on python3 only.

-gmann