Move constraint from install_command to deps

The install_command is used to install os-ken and its dependencies.
However, we don't want to constraint on installing os-ken since
it will fail. Therefore, this commit moves constraint to deps.

Change-Id: Ic4c0d72e9c54e1991cbf390a08b0614f19a2a7af

tox.ini

@@ -5,14 +5,16 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-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}
    PYTHONWARNINGS=default::DeprecationWarning
    OS_STDOUT_CAPTURE=1
    OS_STDERR_CAPTURE=1
    OS_TEST_TIMEOUT=60
-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
 # TODO(hongbin): this is the way Ryu runs unit tests and we inherit
 # this approach as a start. In the future, we should migrate to stestr
 # for consistency with other OpenStack projects.