Issue 358: OpenMCT Plugin support for multiple clients and filtered telemetry

[nttoole]

Refactored the plugin to maintain a set of active-websockets to which realtime telemetry will be sent.
Server also maintains state of web-socket by handling messages: subscribe, unsubscribe, close
Each web-socket maintains a set of subscribed telemetry fields, so only those fields will be transmitted to the web-socket.

We also refactored how the AIT-OpenMct extensions are integrated with OpenMCT. No longer do we download OpenMCT and modify its internal files. Instead, we offer an example web-server that imports openmct as a dependency. This removes an earlier requirement of using an older version of OpenMCT (before they adopted Webpack).

Finally, updated documentation to capture the new approach for deployment.

Testing involved an AIT Config YAML with the following plugin info:

          - plugin:
              name: ait.core.server.plugins.openmct.AITOpenMctPlugin
              inputs:
                - telem_stream
              service_port: 8082
              debug_enabled: True
              database_enabled: False

Testing notes particular to each issue are captured in the issue.

Fixes #357
Fixes #358
Fixes #359

[jasonmlkang]

Tried running ait-server with openmct example server. LGTM!