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.

Also these changes introduce a new explicit env var related to eventlet
testing to avoid misunderstand.

Change-Id: I03b9b179c4cb6ea5a9a8d8c74af3502a0243ace6

tox.ini

@@ -5,7 +5,6 @@ ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
-install_command = pip install {opts} {packages}
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
@@ -13,7 +12,7 @@ deps =
 # We want to support both vanilla stdlib and eventlet monkey patched
 whitelist_externals = env
 commands =
-  lockutils-wrapper stestr run --slowest {posargs}
+  env TEST_EVENTLET=0 lockutils-wrapper stestr run --slowest {posargs}
   env TEST_EVENTLET=1 lockutils-wrapper stestr run --slowest {posargs}
 
 [testenv:pep8]