Honor upper-constraints

Since we don't have many special requirements anymore, we can honor
upper-constraints and avoid some gate breakages.

Cap on pytest is dropped because latest version works just fine on 2.7
and we don't support 2.6 anymore (in master). It gets overwritten by
upper-constraints anyhow.

Change-Id: I173a4db6d14e22cbe11fc790e537b8f226db65f3

test-requirements.txt

@@ -1,5 +1,5 @@
 hacking<0.11,>=0.10.0
-pytest<2.8.0
+pytest
 pytest-cov
 pytest-mock<0.10.0
 pyzabbix==0.7.3

tox.ini

@@ -5,7 +5,7 @@ skipsdist = False
 
 [testenv]
 usedevelop = True
-install_command = pip install -U {opts} {packages}
+install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt