Add SITE_HOST arg to ocs-agent-cli

This makes it easier to have docker-based agents grab their instance
config from a specific host block.

docs/developer/docker.rst

@@ -104,6 +104,10 @@ Environment variables used to configure the Agent are listed in the following ta
    * - ``INSTANCE_ID``
      - ``--instance-id``
      - Agent instance-id, e.g. ``aggregator``
+   * - ``SITE_HOST``
+     - ``--site-host``
+     - Hostname to use when looking up the instance configuration in
+       the Site Config File, e.g. ``host1``.
    * - ``SITE_HUB``
      - ``--site-hub``
      - WAMP server address, e.g. ``ws://10.10.10.10:8001/ws``

ocs/agent_cli.py

@@ -21,10 +21,18 @@
 
   ocs-agent-cli --instance-id INSTANCE_ID
 
-``ocs-agent-cli`` will also inspect environment variables for commonly passed
-arguments to facilitate configuration within Docker containers. Supported
-arguments include, ``--instance-id`` as ``INSTANCE_ID``, ``--site-hub`` as
-``SITE_HUB``, and ``--site-http`` as ``SITE_HTTP``.
+``ocs-agent-cli`` will also inspect environment variables for commonly
+passed arguments to facilitate configuration within Docker containers.
+Those environment variables, if defined and non-trivial, will be
+passed to the agent script unless they are overridden explicitly on
+the ``ocs-agent-cli`` command line.  The environment variables and
+arguments are:
+
+  - ``INSTANCE_ID``, will be the default value for ``--instance-id``
+  - ``SITE_HOST``, for ``--site-host``
+  - ``SITE_HUB``, for ``--site-hub``
+  - ``SITE_HTTP``, for ``--site-http``
+
 
 ``ocs-agent-cli`` relies on the Agent being run belonging to an OCS Plugin. If
 the Agent is not an OCS Plugin it can be run directly using both the
@@ -44,6 +52,7 @@ def _get_parser():
     parser.add_argument('--instance-id', default=None, help="Agent unique instance-id. E.g. 'aggregator' or 'fakedata-1'.")
     parser.add_argument('--site-hub', default=None, help="Site hub address.")
     parser.add_argument('--site-http', default=None, help="Site HTTP address.")
+    parser.add_argument('--site-host', default=None, help="Declare the host the instance is configured in.")
 
     # Not passed through to Agent
     parser.add_argument('--agent', default=None, help="Path to non-plugin OCS Agent.")
@@ -133,7 +142,9 @@ def main(args=None):
     # Format is {"arg name within argparse": "ENVIRONMENT VARIABLE NAME"}
     # E.g. --my-new-arg should be {"my_new_arg": "MY_NEW_ARG"}
     optional_env = {"site_hub": "SITE_HUB",
-                    "site_http": "SITE_HTTP"}
+                    "site_http": "SITE_HTTP",
+                    "site_host": "SITE_HOST",
+                    }
 
     for _name, _var in optional_env.items():
         # Args passed on cli take priority