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.

openstack-discuss thread
http://lists.openstack.org/pipermail/openstack-discuss/2020-April/014237.html

Change-Id: Idda0a78f63d5bc3e8c9e56e042630c8d28db2162

tox.ini

@@ -6,7 +6,6 @@ ignore_basepython_conflict=true
 
 [testenv]
 usedevelop = True
-install_command = pip install -U {opts} {packages}
 basepython = python3
 setenv =
   VIRTUAL_ENV={envdir}