allow optional build parameter to rundeckapp to specify use of a local copy of the grails zip

If the parameter isn't included, the gradle build should fall back to the default behavior of downloading the grails zip.  If the parameter is included, the build should attempt use the zip file (as specified in `rundeckapp/build.gradle` [via `gradle.properties`]) that lives in the provided directory.

```
:core:clean UP-TO-DATE
:plugins:clean UP-TO-DATE
:rundeck-launcher:clean UP-TO-DATE
:rundeckapp:cleanWar UP-TO-DATE
:rundeckapp:downloadGrails
Downloading grails-1.3.7...
:rundeckapp:extractGrails
:rundeckapp:grailsClean SKIPPED
:rundeckapp:clean UP-TO-DATE
```

This comes into play when attempting to build rundeck in isolated/secure environments without access to the internet.