container: Add speedometer family of benchmarks

Create a container to run speedometer benchmarks in Chromium. Selenium
is used to automate driving the browser from Python to navigate to the
benchmark website, run the benchmark and collect the score. Chromium is
run in headless mode in a vanilla debian environment.

Versions 2.0 and 2.1 of the benchmark are supported. Version 3.0 is
causing some issues for automation so will be tackled later.

Benchmark yaml requires only the name, with version encoded in it, e.g:

name: speedometer-v2.1

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>