toaster: browser tests - add Selenium Docker container as driver

toaster: browser tests - add Selenium Docker container as driver

Adds the ability to specify a Selenium Docker container server as
a driver. This allows for repeatable tests independent of host.
Currently we assume you are using the Firefox container. Instructions
are located in the README in tests/browser.

Signed-off-by: brian avery <brian.avery@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>