Make STI script downloading and application sources download executed in parallel

This will speed up downloading script as it will make every download happen in parallel and also it will make `git clone` happen in parallel to that :-)

I also moved templates from `build.go` to `templates.go` as this is what gear is doing and it will make the sources look cleaner.

There is also some refactoring in `build.go`, moving some functions into `util.go` and `selinux.go` to make the `build.go` easier to read.