Fix lower-constraint deps handling

When the lower-constraints tox target was added, it was assumed the
install_command was just running the install and that the dependencies
and constraints were being set using "deps = ".

This fixed the install_command and deps to follow the expected pattern
so the lower-constraints job actual does install the lower constraints.

This also raises the oslo.context minimum as
Ic96c1f1e1a80099d9dafa95a014fc47f05b88e42 added a dependency on a newer
versions kwarg.

Depends-On: https://review.openstack.org/#/c/647726/

Change-Id: I4cc2c3ac158a607b22295c50f83896969a4007ee
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

tox.ini

@@ -7,10 +7,13 @@ skipsdist = True
 usedevelop = True
 whitelist_externals = find
                       rm
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/test-requirements.txt
+deps =
+    -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+    -r{toxinidir}/test-requirements.txt
+    -r{toxinidir}/requirements.txt
 commands =
   rm -f .testrepository/times.dbm
   find . -type f -name "*.py[c|o]" -delete