Update TLS version configuration code. Tests.

Signed-off-by: Joffrey F <joffrey@docker.com>

tests/unit/cli/command_test.py

@@ -2,10 +2,12 @@ from __future__ import absolute_import
 from __future__ import unicode_literals
 
 import os
+import ssl
 
 import pytest
 
 from compose.cli.command import get_config_path_from_options
+from compose.cli.command import get_tls_version
 from compose.config.environment import Environment
 from compose.const import IS_WINDOWS_PLATFORM
 from tests import mock
@@ -46,3 +48,20 @@ class TestGetConfigPathFromOptions(object):
     def test_no_path(self):
         environment = Environment.from_env_file('.')
         assert not get_config_path_from_options('.', {}, environment)
+
+
+class TestGetTlsVersion(object):
+    def test_get_tls_version_default(self):
+        environment = {}
+        assert get_tls_version(environment) is None
+
+    def test_get_tls_version_upgrade(self):
+        environment = {'COMPOSE_TLS_VERSION': 'TLSv1_2'}
+        assert get_tls_version(environment) == ssl.PROTOCOL_TLSv1_2
+
+    def test_get_tls_version_unavailable(self):
+        environment = {'COMPOSE_TLS_VERSION': 'TLSv5_5'}
+        with mock.patch('compose.cli.command.log') as mock_log:
+            tls_version = get_tls_version(environment)
+        mock_log.warn.assert_called_once_with(mock.ANY)
+        assert tls_version is None