Refresh tox
Refresh tox

Add ignore_basepython_conflict [1], this will fix tox warnings about
python versions.
Bump tox minversion to 3.1.0 to ensure that ignore_basepython_conflict
is supported.
Use the default install_commandi [2].

[1] https://tox.readthedocs.io/en/latest/config.html#conf-ignore_basepython_conflict
[2] https://tox.readthedocs.io/en/latest/config.html#conf-install_command

Change-Id: Ic76f25bcd2d70525d02c565449da753c4ba58bc3

openstack/ceilometer,openstack/ceilometer