Use json file for hub config.
Only call configureHub if role = hub.
Merge branch 'hub_config' of https://github.com/smccarthy/Selenium-Grid-Extras into smccarthy-hub_config

Conflicts:
	SeleniumGridExtras/src/main/java/com/groupon/seleniumgridextras/tasks/StartGrid.java
Using the JsonCodec for the json keys
Merge branch 'smccarthy-hub_config'