Unittest the get_split_image_tag function in the docker module

core @ c6522620

-Subproject commit 6be2fbb1c577d34b0dbb51c7338da0b79286658f
+Subproject commit c6522620c562d24031ad32187de83c3768df3c77

test/units/module_tests/TestDocker.py

+import collections
+import mock
+import os
+import unittest
+
+from ansible.modules.core.cloud.docker.docker import get_split_image_tag
+
+class DockerSplitImageTagTestCase(unittest.TestCase):
+
+    def test_trivial(self):
+        self.assertEqual(get_split_image_tag('test'), ('test', 'latest'))
+
+    def test_with_org_name(self):
+        self.assertEqual(get_split_image_tag('ansible/centos7-ansible'), ('ansible/centos7-ansible', 'latest'))
+
+    def test_with_tag(self):
+        self.assertEqual(get_split_image_tag('test:devel'), ('test', 'devel'))
+
+    def test_with_tag_and_org_name(self):
+        self.assertEqual(get_split_image_tag('ansible/centos7-ansible:devel'), ('ansible/centos7-ansible', 'devel'))