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: I16c194c0ee46668e2846ed36763cc14388881ba7

tox.ini

@@ -16,7 +16,6 @@ setenv = VIRTUAL_ENV={envdir}
          OS_TEST_TIMEOUT=60
          OS_TEST_PATH=./cinder/tests/unit
 usedevelop = True
-install_command = pip install {opts} {packages}
 
 deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
         -r{toxinidir}/test-requirements.txt