Support global configuration

Similarly to the way Maven's settings.xml works, provide a global arquillian configuration file located e.g. in ~/.arquillian/arquillian.xml that could be used to configure global machine-specific configuration. This could include:

• container-specific configuration like path to JBoss AS
• credentials (e.g. openshift setup - see https://github.com/jharting/arquillian-container-openshift/blob/master/openshift-express/src/test/resources/arquillian.xml for an example)
• ajocado setup - selenium settings like location of the browser
• others...