validate/launcher: Ensure the HTTP server is started when a pipeline needs it

validate/launcher: Ensure the HTTP server is started when a pipeline needs it

Pipelines declared in gst-integration-testsuites can rely on the validate HTTP
server, so when an URI pointing to it is detected, advertise the server as
needed before starting the test.

For this to work the test scenario should explicitely declare the pipeline uri,
as shown in this example:

    "some_playbin3":
    {
        "pipeline": "playbin3 uri=%(uri)s video-sink=%(videosink)s",
        "config": [
            "%(validateflow)s, pad=sink:sink"
        ],
        "scenarios": ["play_15s"],
        "uri": "http://127.0.0.1:%(http-server-port)s/defaults/html/foo.html"
    }