Update min version of tox
Update min version of tox

This patch bumps the minimum version of tox to 3.18.0 [1].
To incorporate the below changes,
* Remove configuration of "install_command" option in tox, as it
  turns out that this is the default value provided by tox [2].
* The argument such as "whitelist_externals" is deprecated,
  therefore replaces whitelist_externals by allowlist_externals option [1].

[1] https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23
[2] https://tox.readthedocs.io/en/latest/config.html#conf-install_command

Change-Id: Iaf83bbaf7a4dbe03aab42d63a81276cfe42df505

stackforge/tacker,openstack/tacker,stackforge/tacker,openstack/tacker,openstack/tacker