Make the low-constraints job work and modify coverage version

Now this style can't make low-constraints job work, because the pip
will constrain a library to the first item it encounters. Then the
version defined in the uppper file will be installed here.

The solution is to put the upper file in the deps.

Example:
When running low-constraints job:
According to the low-constraints file:
Babel==2.5.3[1] should be installed
Actually the log[2] shows that Babel===2.7.0[3] was installed.

Reference Link:
[1]. https://github.com/openstack/freezer/blob/master/lower-constraints.txt#L6
[2]. http://logs.openstack.org/85/657185/2/check/openstack-tox-lower-constraints/f9d0a50/job-output.txt.gz
[3]. https://releases.openstack.org/constraints/upper/master
[4]. https://review.opendev.org/#/c/601188
[5]. http://lists.openstack.org/pipermail/openstack-discuss/2019-May/006478.html

The coverage version need >=4.5.0 to support py36:
[1]. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=879269
[2]. https://travis-ci.org/agx/git-buildpackage/jobs/534117241

Change-Id: Idf278f4a7aca4470f5996e5166e4724259ef944b

lower-constraints.txt

@@ -11,7 +11,7 @@ cffi==1.11.5
 chardet==3.0.4
 cliff==2.11.0
 cmd2==0.8.1
-coverage==4.0
+coverage==4.5.1
 cryptography==2.1
 ddt==1.0.1
 debtcollector==1.19.0

test-requirements.txt

@@ -3,7 +3,7 @@
 # process, which may cause wedges in the gate later.
 flake8<2.6.0,>=2.5.4 # MIT
 hacking!=0.13.0,<0.14,>=0.12.0 # Apache-2.0
-coverage!=4.4,>=4.0 # Apache-2.0
+coverage>=4.5.1 # Apache-2.0
 ddt>=1.0.1 # MIT
 mock>=2.0.0 # BSD
 pylint==1.9.2 # GPLv2

tox.ini

@@ -5,6 +5,7 @@ skipsdist = True
 [testenv]
 usedevelop = True
 deps =
+    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test-requirements.txt
 
@@ -21,7 +22,7 @@ passenv =
     FREEZER_TEST_NO_LVM
     http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
+install_command = pip install {opts} {packages}
 setenv =
     VIRTUAL_ENV={envdir}
     OS_TEST_PATH = ./freezer/tests/unit