Conditional wrapping of YAML app.

When loading YAML with a single application in the services list don't
create a wrapper application, use the one from the services list.
To explicitly force wrapping use custom attribute "wrapApp: true".

Original commit review notes: https://github.com/neykov/incubator-brooklyn/commit/91851deb7c0cbeb0b1d8d94c7df2afcc33744597