Tasks don't reflect changes to plugin config

[jcooklin]

To reproduce add a plugin configuration item that will be used by a given task, start the given task and then update the configuration item.

Expected result: When a task, depending on a configuration item that is not provided by the task, is changed the task should start using this new configuration item.
Actual result: The new configuration is ignored.

In the example below:

1. snapd is started with the following config.
2. Plugins snap-plugin-collector-mock1 and snap-plugin-publisher-mock-file are loaded.
3. The task is left to run for 5 seconds.
4. The plugin-config for mock-file is updated

• curl -X PUT --data '{"file": "/tmp/snap/snap-file2.out"}' http://localhost:8181/v1/plugins/publisher/mock-file/3/config

---
control:
  plugins:
    publisher:
      mock-file:
        all:
          file: /tmp/snap/snap-file.out
          test: true

[geauxvirtual]

What's happening here is when a new task is created, the workflow ends up being a pointer to the workflow created from the wfmap here, https://github.com/intelsdi-x/snap/blob/master/scheduler/scheduler.go#L315-L330.

Further in createTask, the code tries to validate the dependencies for the plugins, https://github.com/intelsdi-x/snap/blob/master/scheduler/scheduler.go#L342.

When this occurs, any global plugin config defined gets merged into the workflow here, https://github.com/intelsdi-x/snap/blob/master/control/subscription_group.go#L238-L240.

Since task.workflow is the same pointer to wf, task.workflow now has configuration items defined for the plugin that came from the plugin global config and not the task manifest.

When PublishMetrics is called, any global plugin config is pulled in, and then any task level config for the plugin is merged in with the global plugin config, with the task level config taking precedence. https://github.com/intelsdi-x/snap/blob/master/control/control.go#L986-L993.