Add graphs templates

[falzm]

Implement a templating system to allow dynamic graphs rendering.

The idea is to be able to display collections featuring graphs based on a pre-defined "model", but without requiring explicitly defined graphs features such as sources and metrics.

[vbatoufflet]

Maybe we could use something like the following JSON structure to store template relations with their attributes:

{
    "id": "...",
    "template": "...",
    "attributes": {
        "source1": "host1.example.net"
    }
}

[falzm]

Following the same logic, a graph template JSON definition would look like this:

{
    "id": "85c08784-d8c0-4366-7c1b-70dbefde6bed",
    "name": "{{ .Source }} — network traffic",
    "description": "",
    "type": 1,
    "stack_mode": 1,
    "unit_type": 2,
    "unit_legend": "bytes/s",
    "groups": [
        {
            "name": "group0",
            "type": 4,
            "stack_id": 0,
            "series": [
                {
                    "name": "group:net.*.octets.rx",
                    "origin": "collectd",
                    "source": "{{ .Source }}",
                    "metric": "group:net.*.octets.rx",
                    "options": null
                },
                {
                    "name": "group:net.*.octets.tx",
                    "origin": "collectd",
                    "source": "{{ .Source }}",
                    "metric": "group:net.*.octets.tx",
                    "options": null
                }
            ],
            "options": {}
        }
    ]
}

[vbatoufflet]

Graphs templates are in the process of being shipped soon and we are working on collection template support.

See facette:templates for details.

[falzm]

A dedicated issue (#170) has been opened for collection templates implementation, since it will be released in a separate version.