GRAILS_HOME does not work when using more than one Grails version

[robertoschwald]

Setting GRAILS_HOME as the fallback env var does not work if using more than one Grails versions. Especially Spring-Loaded Classes for Grails 3 can lead to all sort of Problems (e.g. in Intellij IDEA) if GRAILS_HOME is set to a Grails 2.x version.

I provide a PR which supports an alternate env. var "${TOOL}_DEFAULT_HOME" which does not interfere with the problem and is used if ${TOOL}_HOME is not set.