[ADD] base_external_system: Implement interface/adapter (#993)

* [ADD] base_external_system: Implement interface/adapter for external systems

* base_external_system: Fix OS model, add inherits, add validate

* base_external_system: Usability and private key pass

* base_external_system: Use contextmanager in adapter client

* base_external_system: Move contextmanager to interface

* base_external_system: Include contextmanager on adapter and system

* base_external_system: Unify client

* Use password widget for password field

* Add tests & security

* Fix lint

* Add plaintext note

base_external_system/tests/test_external_system_os.py

@@ -0,0 +1,40 @@
+# -*- coding: utf-8 -*-
+# Copyright 2017 LasLabs Inc.
+# License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl).
+
+import os
+
+from .common import Common
+
+
+class TestExternalSystemOs(Common):
+
+    @classmethod
+    def setUpClass(cls):
+        """Remember the working dir, just in case."""
+        super(TestExternalSystemOs, cls).setUpClass()
+        cls.working_dir = os.getcwd()
+
+    @classmethod
+    def tearDownClass(cls):
+        """Set the working dir back to origin, just in case."""
+        super(TestExternalSystemOs, cls).tearDownClass()
+        os.chdir(cls.working_dir)
+
+    def setUp(self):
+        super(TestExternalSystemOs, self).setUp()
+        self.record = self.env.ref('base_external_system.external_system_os')
+
+    def test_external_get_client_returns_os(self):
+        """It should return the Pyhton OS module."""
+        self.assertEqual(self.record.external_get_client(), os)
+
+    def test_external_get_client_changes_directories(self):
+        """It should change to the proper directory."""
+        self.record.external_get_client()
+        self.assertEqual(os.getcwd(), self.record.remote_path)
+
+    def test_external_destroy_client_changes_directory(self):
+        """It should change back to the previous working directory."""
+        self.record.external_destroy_client(None)
+        self.assertEqual(os.getcwd(), self.working_dir)