This patch makes java files are copied to the guest image only when
'java' modue is included.

Modules can pull it explicitly by stating:

  require('java')

or implicitly, by creating api.run_java() run configurations.
In future we could consider moving api.run_java() into a java
meta-module.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>