Stop configuring install_command in tox.
Stop configuring install_command in tox.

It turns out that this is the the default value provided by tox:

https://tox.readthedocs.io/en/latest/config.html#conf-install_command

So we can remove the line and simply use the default value.

Change-Id: I93b6aea505bc0b4e3e91229e7a2f35a8c01db6d7

openstack/os-brick,openstack/os-brick