In the code to retrieve the variables from the configuration files we
need to pass a execution callback, this callback will be called in a
goroutine. This callback can be executed multiple time until the
composable renderer is stopped. There were a problem in the code that
made the callback called multiple time and it made the waitgroup
internal counter to do to a negative values.

This commit change the behavior, it start the composable renderer give
it a callback when the callback receives the variables it will stop the
composable's Run method using the context.

This ensure that the callback will be called a single time and that the
variables are correctly retrieved.

Fixes: elastic#806