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

Bumping l-c:
-openstacksdk to 0.13.0
-Werkzeug to 0.14

Bumping t-r:
-openstacksdk to 0.13.0

Change-Id: I6e7e3792f2cede28ea9098ba546f415cec0ddf9b

lower-constraints.txt

@@ -26,7 +26,7 @@ mock==2.0.0
 mox3==0.20.0
 munch==2.1.0
 openstackdocstheme==1.20.0
-openstacksdk===0.11.2
+openstacksdk===0.13.0
 oslotest==3.2.0
 pbr==2.0.0
 Pygments==2.2.0
@@ -54,4 +54,4 @@ testtools==2.2.0
 traceback2==1.4.0
 unittest2==1.1.0
 urllib3==1.21.1
-Werkzeug==0.9
+Werkzeug==0.14

test-requirements.txt

@@ -11,7 +11,7 @@ python-subunit>=1.0.0 # Apache-2.0/BSD
 sphinx!=1.6.6,!=1.6.7,>=1.6.2 # BSD
 openstackdocstheme>=1.20.0 # Apache-2.0
 # used by nova driver
-openstacksdk>=0.11.2  # Apache-2.0
+openstacksdk>=0.13.0  # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 stestr>=1.0.0 # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD

tox.ini

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