Use env to find python3
Use env to find python3

Our container doesn't have python3 at /usr/bin/python3. This
changes the script so that /usr/bin/env is used to find python3.

For discussion/opinions about how (and the better way) to locate
python3:

- https://developpaper.com/the-difference-between-usr-bin-python-and-usr-bin-env-python-in-python/
- https://stackoverflow.com/questions/13879569/usr-bin-python-and-usr-bin-env-python-which-support

Change-Id: I88cab53244913b5c9bfce6729f38f93a57ddbe80

openstack/ironic,openstack/ironic