Fix ansible-test docker python version handling.

This removes the old name based version detection behavior and
uses versions defined in the docker completion file instead, as
the new containers do not follow the old naming scheme.

(cherry picked from commit 54937ba784)

test/runner/completion/docker.txt

@@ -1,11 +1,11 @@
-default name=quay.io/ansible/default-test-container:1.2.0
+default name=quay.io/ansible/default-test-container:1.2.0 python=3
 centos6 name=quay.io/ansible/centos6-test-container:1.4.0 seccomp=unconfined
 centos7 name=quay.io/ansible/centos7-test-container:1.4.0 seccomp=unconfined
 fedora24 name=quay.io/ansible/fedora24-test-container:1.4.0 seccomp=unconfined
 fedora25 name=quay.io/ansible/fedora25-test-container:1.4.0 seccomp=unconfined
-fedora26py3 name=quay.io/ansible/fedora26py3-test-container:1.4.0
-fedora27py3 name=quay.io/ansible/fedora27py3-test-container:1.4.0
+fedora26py3 name=quay.io/ansible/fedora26py3-test-container:1.4.0 python=3
+fedora27py3 name=quay.io/ansible/fedora27py3-test-container:1.4.0 python=3
 opensuse42.3 name=quay.io/ansible/opensuse42.3-test-container:1.4.0 seccomp=unconfined
 ubuntu1404 name=quay.io/ansible/ubuntu1404-test-container:1.4.0 seccomp=unconfined
 ubuntu1604 name=quay.io/ansible/ubuntu1604-test-container:1.4.0 seccomp=unconfined
-ubuntu1604py3 name=quay.io/ansible/ubuntu1604py3-test-container:1.4.0 seccomp=unconfined
+ubuntu1604py3 name=quay.io/ansible/ubuntu1604py3-test-container:1.4.0 seccomp=unconfined python=3

test/runner/lib/executor.py

@@ -52,6 +52,7 @@ from lib.util import (
     get_available_port,
     generate_pip_command,
     find_python,
+    get_docker_completion,
 )
 
 from lib.docker_util import (
@@ -1457,15 +1458,9 @@ def get_integration_docker_filter(args, targets):
             display.warning('Excluding tests marked "%s" which require --docker-privileged to run under docker: %s'
                             % (skip.rstrip('/'), ', '.join(skipped)))
 
-    docker_image = args.docker.split('@')[0]  # strip SHA for proper tag comparison
-
     python_version = 2  # images are expected to default to python 2 unless otherwise specified
 
-    if docker_image.endswith('py3'):
-        python_version = 3  # docker images ending in 'py3' are expected to default to python 3
-
-    if docker_image.endswith(':default'):
-        python_version = 3  # docker images tagged 'default' are expected to default to python 3
+    python_version = int(get_docker_completion().get(args.docker_raw).get('python', str(python_version)))
 
     if args.python:  # specifying a numeric --python option overrides the default python
         if args.python.startswith('3'):