Update TOX/UPPER_CONSTRAINTS_FILE for stable/train

Update the URL to the upper-constraints file to point to the redirect
rule on releases.openstack.org so that anyone working on this branch
will switch to the correct upper-constraints list automatically when
the requirements repository branches.

Until the requirements repository has as stable/train branch, tests will
continue to use the upper-constraints list on master.

Change-Id: I0abb4375f76549f4b06d0acfa257e754955736f7

.gitreview

@@ -2,3 +2,4 @@
 host=review.opendev.org
 port=29418
 project=openstack/python-watcherclient.git
+defaultbranch=stable/train

tox.ini

@@ -11,7 +11,7 @@ install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/train}
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 whitelist_externals =
@@ -43,7 +43,7 @@ commands =
 [testenv:docs]
 basepython = python3
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/train}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 commands = python setup.py build_sphinx