Force kolla-ansible to use python3 on target hosts

Some kolla-ansible tasks still use the unversioned
python symlink and they end up using python2 on
target hosts. This has been fixed in upstream master
branch [1]. But the fix is not available yet in the
stable/train branch.

We only capture pip3 dependencies during packaging
and hence during offline installation we will not
be able run those tasks. This patchset will force
kolla-ansible to use python3 interpreter.

[1]: https://review.opendev.org/#/c/713283/

Change-Id: I0bad5840f84a561aae88d77568d69db8c2f06ac6
3 files changed