[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/models/external_system_os.py

@@ -0,0 +1,43 @@
+# -*- coding: utf-8 -*-
+# Copyright 2017 LasLabs Inc.
+# License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl.html).
+
+import os
+
+from odoo import api, models
+
+
+class ExternalSystemOs(models.Model):
+    """This is an Interface implementing the OS module.
+
+    For the most part, this is just a sample of how to implement an external
+    system interface. This is still a fully usable implementation, however.
+    """
+
+    _name = 'external.system.os'
+    _inherit = 'external.system.adapter'
+    _description = 'External System OS'
+
+    previous_dir = None
+
+    @api.multi
+    def external_get_client(self):
+        """Return a usable client representing the remote system."""
+        super(ExternalSystemOs, self).external_get_client()
+        if self.system_id.remote_path:
+            self.previous_dir = os.getcwd()
+            os.chdir(self.system_id.remote_path)
+        return os
+
+    @api.multi
+    def external_destroy_client(self, client):
+        """Perform any logic necessary to destroy the client connection.
+
+        Args:
+            client (mixed): The client that was returned by
+             ``external_get_client``.
+        """
+        super(ExternalSystemOs, self).external_destroy_client(client)
+        if self.previous_dir:
+            os.chdir(self.previous_dir)
+            self.previous_dir = None