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 requirements and l-c with:
keystoneauth1 3.18.0
alembic 0.9.6
pika 0.10.0 (necessary for installation)
Change-Id: I849342bd077ab2f8bbda4e0a228c308eb7a4908b

lower-constraints.txt

@@ -1,4 +1,4 @@
-alembic==0.8.10
+alembic==0.9.6
 automaton==1.9.0
 Babel==2.3.4
 bandit==1.1.0
@@ -14,7 +14,7 @@ hacking==3.0.0
 ironic-lib==2.17.0
 jsonpath-rw==1.2.0
 jsonschema==2.6.0
-keystoneauth1==3.14.0
+keystoneauth1==3.18.0
 keystonemiddleware==4.18.0
 mock==3.0.0
 netaddr==0.7.18
@@ -36,6 +36,7 @@ oslo.service==1.24.0
 oslotest==3.2.0
 oslo.utils==3.33.0
 pbr==2.0.0
+pika==0.10.0
 pymemcache==1.2.9
 pytz==2013.6
 reno==2.5.0

requirements.txt

@@ -2,7 +2,7 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 automaton>=1.9.0 # Apache-2.0
-alembic>=0.8.10 # MIT
+alembic>=0.9.6 # MIT
 construct>=2.9.39 # MIT
 eventlet!=0.18.3,!=0.20.1,>=0.18.2 # MIT
 Flask>=1.0 # BSD
@@ -10,7 +10,7 @@ futurist>=1.2.0 # Apache-2.0
 ironic-lib>=2.17.0 # Apache-2.0
 jsonpath-rw<2.0,>=1.2.0 # Apache-2.0
 jsonschema>=2.6.0 # MIT
-keystoneauth1>=3.14.0 # Apache-2.0
+keystoneauth1>=3.18.0 # Apache-2.0
 keystonemiddleware>=4.18.0 # Apache-2.0
 netaddr>=0.7.18 # BSD
 pbr!=2.1.0,>=2.0.0 # Apache-2.0

tox.ini

@@ -6,7 +6,6 @@ ignore_basepython_conflict=true
 [testenv]
 basepython = python3
 usedevelop = True
-install_command = pip install -U {opts} {packages}
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt