Update the minversion parameter.
Update the minversion parameter.

Update the minversion parameter to use the python -m pip to install
python packages:

https://tox.readthedocs.io/en/latest/changelog.html#id185

It's recommend to use this. Remove the useless install_command
parameter.

Change-Id: Iebc9d821a7f732b44c61c69337dc9f305aa70658

openstack/python-solumclient