Migrate grenade jobs to py3
Migrate grenade jobs to py3

As part of community goal of dropping py27 support[1], we are
moving the devstack to py3 by default[2]. That will make grenade job
to perform upgrade from py2 to py3 which will not work (we have seen
the failure in neutron-grenade job).

To avoid existing grenade py2 job break, this commit moves grenade
jobs to py3 which is what we planned as part dropping the py2 support.

[1] https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html
[2] https://review.opendev.org/#/c/649097/12
    http://lists.openstack.org/pipermail/openstack-discuss/2019-November/010938.html

Change-Id: I1a0018dd164ccdab803b67c5883d3f28819bd45d

openstack/sahara,openstack/sahara