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

Currently, we are overriding 'install_command' to use 'pip'. This is
considered poor behavior and 'python -m pip' should be used instead:

https://snarky.ca/why-you-should-use-python-m-pip/

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: I2bf8b8509788fdc2f23c9ddf4211b19de7b3fb87

mahak/keystone,mahak/keystone,mahak/keystone,openstack/keystone,openstack/keystone,openstack/keystone