Fixes # 820

Summary of changes:

• Fixes issue where we override the plugin for a given metric

Testing done:

• Updated workflow test to cover this case
• Manual

Scenario:

• Start snap and load 2 different versions of the same plugin
  • The lesser version should contain a metric that is not available in the new version
• Load a task that retrieves a metric that is only available in the older plugin

Snapd config which causes /intel/mock/test to become available:

---
control:
  plugins:
    collector:
      mock:
        versions:
          1:
            test: true

Task definition:

---
  version: 1
  schedule:
    type: "simple"
    interval: "1s"
  workflow:
    collect:
      metrics:
        /intel/mock/bar: {}
        /intel/mock/foo: {}
        /intel/mock/test: {}
      config:
        /intel/mock:
          user: "root"
          password: "secret"

Before:

After:

@intelsdi-x/snap-maintainers