Fixes #840 Add Url to HREF field for available plugin

[IRCody]

Fixes #840

Summary of changes:

• Added url to HREF field for available plugins returned from the rest api.

Testing done:

• manual testing

 "available_plugins": [
      {
        "name": "mock",
        "version": 2,
        "type": "collector",
        "hitcount": 12,
        "last_hit_timestamp": 1460053388,
        "id": 1,
        "href": "http://localhost:8181/v1/plugins/collector/mock/2"
      }
    ]

@intelsdi-x/snap-maintainers

[candysmurf]

👍 LGTM