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: I22eb2dc7935b529301c3608ac81d6b60998014fb

lower-constraints.txt

@@ -7,7 +7,7 @@ certifi==2018.1.18
 cffi==1.11.5
 chardet==3.0.4
 coverage==4.0
-cryptography==2.1.4
+cryptography==2.2.1
 debtcollector==1.19.0
 doc8==0.6.0
 docutils==0.14

tox.ini

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