Currently, we are overriding 'install_command' to use 'pip'. This is
considered poor behavior and 'python -m pip' should be used instead:
It turns out that this is the the default value provided by tox:
So we can remove the line and simply use the default value.
This change showed we needed a newer version of debtcollector in lower
constraints, aliging with the version required by os-vif. This change
showed we needed also a newer version of cffi and keystoneauth1 in lower
constraints. Update also the requirements file for debtcollector and