Browsertests: make config variables environment variables

Currently some configuration needs to be set in config.yml while other
configuration is done by setting environment variables.

This change makes it possible to fully specify your configuration by
either using environment variables or a config.yml. Note that if an
environment variable is set, the configuration is taken from there while
when it is not set, it will be taken from config.yml.

Change-Id: I05a52308bc3908804eedcc4d4f861613272a6467