#8 support alternate env var ${TOOL}_DEFAULT_HOME if ${TOOL}_HOME cant be used

[robertoschwald]

#8 In case you use many different Grails versions for different projects, you might not be able to use a fixed GRAILS_HOME env var, as this can lead to clashes (e.g. if your use Grails 2 and Grails 3 projects). Therefore I added another env var named ${TOOL}_DEFAULT_HOME to define e.g. the default Grails version to use without clashing with other Grails versions used.

e.g. if you set GRAILS_DEFAULT_HOME="/x/y/z/grails-2.4.4", this version is used if no version can be obtained.