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 the following libs in lower-constraints to avoid version
conflict:
-osprofiler 2.3.0
-python-keystoneclient to 3.22.0
-python-cinderclient to 5.0.0

Change-Id: Id0049dea1e16f9c459e7e63e5f7d96666fff708a

lower-constraints.txt

@@ -59,7 +59,7 @@ oslo.policy==1.34.0
 oslo.serialization==2.25.0
 oslo.utils==3.36.0
 oslotest==3.2.0
-osprofiler==2.0.0
+osprofiler==2.3.0
 packaging==17.1
 pbr==2.0.0
 pep8==1.5.7
@@ -73,10 +73,10 @@ pyOpenSSL==17.5.0
 pyparsing==2.2.0
 pyperclip==1.6.0
 pyScss==1.3.7
-python-cinderclient==3.5.0
+python-cinderclient==5.0.0
 python-glanceclient==2.9.1
 python-ironicclient==2.3.0
-python-keystoneclient==3.15.0
+python-keystoneclient==3.22.0
 python-mimeparse==1.6.0
 python-neutronclient==6.7.0
 python-novaclient==10.1.0

tox.ini

@@ -6,7 +6,6 @@ ignore_basepython_conflict=true
 
 [testenv]
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 basepython = python3
 setenv = VIRTUAL_ENV={envdir}
          NOSE_WITH_OPENSTACK=1
@@ -15,7 +14,8 @@ setenv = VIRTUAL_ENV={envdir}
          NOSE_OPENSTACK_YELLOW=0.025
          NOSE_OPENSTACK_SHOW_ELAPSED=1
          DJANGO_SETTINGS_MODULE=ironic_ui.test.settings
-deps = -r{toxinidir}/requirements.txt
+deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
    {toxinidir}/manage.py test ironic_ui --settings=ironic_ui.test.settings --exclude-tag integration