Update tox.ini for new upper constraints strategy

The requirements team has defined a new strategy for handling
upper constraints[1]. This patch applies those strategies to
Octavia.

[1] http://lists.openstack.org/pipermail/openstack-discuss/ \
    2019-May/006478.html

Change-Id: I38a5bedbdd7852bd6e7bfff4917634458a6edbf3

tox.ini

@@ -5,11 +5,12 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt} {opts} {packages}
+install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
-deps = -r{toxinidir}/test-requirements.txt
+deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/test-requirements.txt
 commands =
   stestr run {posargs}
   stestr slowest
@@ -40,7 +41,7 @@ commands =
 [testenv:docs]
 basepython = python3
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/doc/requirements.txt
 whitelist_externals = rm
@@ -51,7 +52,7 @@ commands =
 [testenv:releasenotes]
 basepython = python3
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/doc/requirements.txt
 commands =