Fix typo for USE_PYTHON3
Fix typo for USE_PYTHON3

DevStack best practice seems to be to use True/False with
the first char upper case, so let's use True (instead
of true)

Change-Id: I6b13fbaf6f50e239169a8792e65863c676e87820

openstack-infra/project-config,openstack-infra/project-config