add source field to Meter model

[Guangyu Suo]

Just like Sample model, I think we should add source filed to Meter model to identify where the meter comes from

http://localhost:8777/v2/meters/image

[
    {
        "counter_name": "image",
        "user_id": null,
        "resource_id": "63751da0-1dd2-4db6-89e7-b143c56f8cc0",
        "timestamp": "2013-09-15T03:31:47",
        "resource_metadata": {
            "status": "active",
            "name": "cirros-0.3.1-x86_64-uec",
            "deleted": "False",
            "checksum": "f8a2eeee2dc65b3d9b6e63678955bd83",
            "created_at": "2013-09-15T00:32:23",
            "disk_format": "ami",
            "updated_at": "2013-09-15T00:32:23",
            "properties.ramdisk_id": "6b4e170a-456c-4854-942b-8f0b8f54dba4",
            "properties.kernel_id": "b4a3a1c4-c0a9-4b34-b3e3-084a1756cfd9",
            "protected": "False",
            "container_format": "ami",
            "min_disk": "0",
            "is_public": "True",
            "deleted_at": "None",
            "min_ram": "0",
            "size": "25165824"
        },
        "source": "openstack",
        "counter_unit": "image",
        "counter_volume": 1,
        "project_id": "a3eb8aa95173470bb5e52670fdc39795",
        "message_id": "5a046802-1db7-11e3-b5ea-689423ac42ad",
        "counter_type": "gauge"
    },
    .......
]

http://localhost:8777/v2/meters
[
    {
        "user_id": null,
        "name": "image",
        "resource_id": "27d447c7-7707-48e2-8c90-8b83797b7e89",
        "meter_id": "MjdkNDQ3YzctNzcwNy00OGUyLThjOTAtOGI4Mzc5N2I3ZTg5K2ltYWdl\n",
        "project_id": "a3eb8aa95173470bb5e52670fdc39795",
        "type": "gauge",
        "unit": "image"
    },
    .......
]

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/46639

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/46639
Committed: http://github.com/openstack/ceilometer/commit/c91381444531145149f684860caf6c4ac984c6aa
Submitter: Jenkins
Branch: master

commit c91381444531145149f684860caf6c4ac984c6aa
Author: Guangyu Suo <email address hidden>
Date: Sun Sep 15 12:21:18 2013 +0800

    Add source field to Meter model

    Just like Sample model, we should also add source field to
    Meter model to identify where the meter comes from.

    Change-Id: I2381d67f84692bfc7d167c2b0d90bfa32b845f00
    Fixes: Bug #1225583