Add support for specifying panels at dashboard construction time

[dlespiau]

At the moment the API looks like:

const dashboard = name => new g.Dashboard(`Service > ${name}`)
  .addPanel(
    new g.Graph(`${name} RPS`, {
      datasource: '$datasource',
    })
      .addTargets([
        new g.Prometheus(RPS(job(name)), { legendFormat: '{{code}}' }),
      ]),
  );

We should allow construction time panels = [ ... ] for people that prefer that option:

const dashboard = name => new g.Dashboard(`Service > ${name}`, {
  panels = [
    new g.Graph(`${name} RPS`, {
      datasource: '$datasource',
      targets = [
        new g.Prometheus(RPS(job(name)), { legendFormat: '{{code}}' }),
      ];
    })
  ].
});