Add 'webperf' module; provision asset-check service

This patch adds a 'webperf' module which I plan to grow into a set of scripts
and configs for keeping tabs on front-end performance. A follow-up patch will
migrate the Navigation Timing Puppet code to this module. Although Puppet is
not a Deployment System™, the scripts involved are usually quite small and
benefit from living in close proximity to the Puppet code that provisions them.
Aggregating these scripts in a proper repository or package is something I'd
like to do in the future, but right now I don't have a firm sense of what this
will grow into, so I don't want to do that yet.

The webperf module included in this patch comes with a single tool that I wrote
called 'asset-check'. It connects to a site using PhantomJS (a headless WebKit
platform) and keeps track of the number of resources that had to be fetched in
the course of rendering the page and their byte sizes. PhantomJS is required
because many of the asset requests on Wikimedia sites are constructed by
JavaScript code, and so merely cURLing the page won't do -- one has to actually
parse and execute its JavaScript payload in order to assess its total size.

A wrapper script implemented in Python invokes PhantomJS with asset-check.js
for a set of URLs representing the main pages of popular Wikimedia wikis. The
script repeats this every five minutes, reporting the measurements to Ganglia
by shelling out to Gmetric.

The payload size of wiki main pages don't change minute-to-minute, so it's a
bit silly to poll so often (i.e., every five minutes). At the same time,
Ganglia tends to choke on longer intervals, and I'd really like to have this
data in Ganglia. I don't think the hourly or daily views of this data will be
interesting, but the weekly / monthly / yearly views will be.

Because the scripts are small and simple, the configuration is hard-coded in
the script source. This isn't especially nice, but I'll clean it up as I
refactor this module into a proper suite of tools.

Change-Id: I8844e1941a5a5ceb207ebd92ad11f1e81618584d
Merge "Add 'webperf' module; provision asset-check service" into production