Add additional Gradle Configuration support in Forge

[jamesfredley]

Additional Gradle Scopes

integrationTest* - Geb, Selenium and perhaps some others should preferably be moved to those.

api - for plugins

can be added after apache/grails-forge#441

[jamesfredley]

apache/grails-forge#424 (comment)

[jdaugherty]

Shouldn't Geb / Selenium be considered a functional test? An integration isn't necessarily end-to-end using the stack, but functional is.