ansible 2.5.1 breaks testing with pytest-xdist

2.5.0 works fine. Here's the error:
```
$ tox -rv -e integration test_inventory
...
integration installed: ansible==2.5.1,apipkg==1.4,asn1crypto==0.24.0,attrs==18.1.0,bcrypt==3.1.4,cffi==1.11.5,cryptography==2.2.2,enum34==1.1.6,execnet==1.5.0,funcsigs==1.0.2,idna==2.6,ipaddress==1.0.22,Jinja2==2.10,MarkupSafe==1.0,more-itertools==4.1.0,paramiko==2.4.1,pluggy==0.6.0,py==1.5.3,pyasn1==0.4.2,pycparser==2.18,PyNaCl==1.2.1,pytest==3.5.1,pytest-forked==0.2,pytest-xdist==1.22.2,PyYAML==3.12,six==1.11.0,testinfra==1.12.0
integration runtests: PYTHONHASHSEED='537973470'
...
plugins: testinfra-1.12.0, xdist-1.22.2, forked-0.2
==================================== ERRORS ====================================
____________________________ ERROR collecting gw14 _____________________________
Different tests were collected between gw11 and gw14. The difference is:
--- gw11

+++ gw14

@@ -1,7 +1,7 @@

+roles/ceph-collectd/tests/test_collectd.py::TestCollectd::()::test_service_enabled[ansible://mira001.front.sepia.ceph.com]
+roles/ceph-collectd/tests/test_collectd.py::TestCollectd::()::test_logfile_present[ansible://mira001.front.sepia.ceph.com]
roles/ceph-collectd/tests/test_collectd.py::TestCollectd::()::test_service_enabled[ansible://mira058.front.sepia.ceph.com]
roles/ceph-collectd/tests/test_collectd.py::TestCollectd::()::test_logfile_present[ansible://mira058.front.sepia.ceph.com]
-roles/ceph-collectd/tests/test_collectd.py::TestCollectd::()::test_service_enabled[ansible://mira001.front.sepia.ceph.com]
-roles/ceph-collectd/tests/test_collectd.py::TestCollectd::()::test_logfile_present[ansible://mira001.front.sepia.ceph.com]
roles/ceph-collectd/tests/test_collectd.py::TestCollectd::()::test_service_enabled[ansible://mira078.front.sepia.ceph.com]
roles/ceph-collectd/tests/test_collectd.py::TestCollectd::()::test_logfile_present[ansible://mira078.front.sepia.ceph.com]
roles/ceph-docker/tests/test_docker.py::TestDocker::()::test_docker_running[ansible://mira053.front.sepia.ceph.com]
@@ -16,10 +16,10 @@

roles/ceph-graphite/tests/test_graphite.py::TestGraphite::()::test_ports_open[ansible://mira053.front.sepia.ceph.com-tcp-0.0.0.0-8080]
roles/ceph-mgr/tests/test_mgr.py::TestMgr::()::test_prometheus_module[ansible://mira058.front.sepia.ceph.com]
roles/ceph-mgr/tests/test_mgr.py::TestMgr::()::test_prometheus_module[ansible://mira001.front.sepia.ceph.com]
+roles/ceph-node-exporter/tests/test_node_exporter.py::TestNodeExporter::()::test_service_enabled[ansible://mira001.front.sepia.ceph.com]
+roles/ceph-node-exporter/tests/test_node_exporter.py::TestNodeExporter::():
2018-05-10T00:32:21.232 INFO:teuthology.task.ansible.tests::test_port_open[ansible://mira001.front.sepia.ceph.com]
roles/ceph-node-exporter/tests/test_node_exporter.py::TestNodeExporter::()::test_service_enabled[ansible://mira058.front.sepia.ceph.com]
roles/ceph-node-exporter/tests/test_node_exporter.py::TestNodeExporter::()::test_port_open[ansible://mira058.front.sepia.ceph.com]
-roles/ceph-node-exporter/tests/test_node_exporter.py::TestNodeExporter::()::test_service_enabled[ansible://mira001.front.sepia.ceph.com]
-roles/ceph-node-exporter/tests/test_node_exporter.py::TestNodeExporter::()::test_port_open[ansible://mira001.front.sepia.ceph.com]
roles/ceph-node-exporter/tests/test_node_exporter.py::TestNodeExporter::()::test_service_enabled[ansible://mira078.front.sepia.ceph.com]
roles/ceph-node-exporter/tests/test_node_exporter.py::TestNodeExporter::()::test_port_open[ansible://mira078.front.sepia.ceph.com]
roles/ceph-prometheus/tests/test_prometheus.py::TestPrometheus::()::test_port_open[ansible://mira053.front.sepia.ceph.com]
```