plugin refresh does not behave correctly

[edaniszewski]

It was discovered that the plugin refresh mechanics are not working correctly. What was done to discover this was:

• create a plugin, have it be registered with synse
• check the /plugin endpoint and see it there and active
• terminate the plugin
• check the /plugin endpoint, see that it is now inactive
• start a new set of plugins

The new set of plugins does not show up in the /plugin output because of various checks/assumptions gating the plugin refresh.

To fix this, I believe two things can be added:

• a refresh query param on the plugin endpoint -- this should allow a manual triggering of plugins to be refreshed. The API docs should be updated accordingly.
• a background task to periodically refresh plugins. This would likely also need a config value to set the refresh rate.

[edaniszewski]

work for this complete in the above referenced PR