[CI] Add upper constraints to install command

In cases when a package is installed in tox's develop-inst phase upper
constraints are not used if it were not defined in 'deps' of the tox
target. This caused for example 'tox -e docs' to fail in case a package
introduced some branch incompatible change.

There are two option to fix this:
* add the upper constraints to 'deps' of the target (and pre-install
  requirements for some cases) OR
* use it in the install_command

In the past the latter needed some extra attention (due to e.g. lower
constraints tests) and projects were encouraged to remove the
install_command, but nowadays this seems better fitting for some
project and easier to use. So this patch fixes the upper constraints
usage via the custom install_command and by removing it from 'deps'.

Change-Id: Iae07e1c5abde09874928951ac55c2d0355bcd244

tox.ini

@@ -24,9 +24,8 @@ setenv =
 # clue for oslo.db to use file-based database.
   OS_TEST_DBAPI_ADMIN_CONNECTION=sqlite:////tmp/placeholder-never-created-nor-used.db
 usedevelop = True
-install_command = pip install {opts} {packages}
-deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
-       -r{toxinidir}/test-requirements.txt
+install_command = python -m pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
+deps = -r{toxinidir}/test-requirements.txt
        -r{toxinidir}/requirements.txt
 commands =
     find . -type f -name "*.pyc" -delete
@@ -103,7 +102,6 @@ commands =
 # This environment is called from CI scripts to test and publish
 # the API Ref to docs.openstack.org.
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/doc/requirements.txt
 commands =
   rm -rf api-ref/build
@@ -151,7 +149,6 @@ paths = ./glance/hacking
 
 [testenv:docs]
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/doc/requirements.txt
 commands =
   rm -fr doc/build
@@ -166,7 +163,6 @@ commands = {posargs}
 
 [testenv:releasenotes]
 deps =
-    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html