Add support for enabling JVM -Xrs flag on all ANT tasks

Add support for automatically specifying the JVM's -Xrs flag for all ANT tasks. See http://docs.oracle.com/javase/6/docs/technotes/tools/windows/java.html for details on the -Xrs flag.

This issue is in response to a support request: https://support.atlassian.com/browse/BSP-7198

If Bamboo is hosted on Windows, and an administrator logs in to perform maintenance via Remote Desktop, any running ANT tasks are interrupted when he logs out. This is a problem with the JVM, where it misinterprets a logoff signal from Windows and erroneously terminates running Java processes (see http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6871190).

This problem can be avoided by specifying the -Xrs flag when the JVM is called. In Bamboo, adding ANT_OPTS="-Xrs" to the Environment Variables for an ANT task accomplishes this.

However this is problematic because users must remember to manually specify this every time they create new ANT tasks. Bamboo should support this flag automatically as some form of configuration parameter, so that it does not need to be entered per task.