Fix a panic with wg passed to the composable object

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

internal/pkg/agent/install/uninstall.go

@@ -235,9 +235,15 @@ func applyDynamics(ctx context.Context, log *logger.Logger, cfg *config.Config)
 		varsArray := make([]*transpiler.Vars, 0)
 		var wg sync.WaitGroup
 		wg.Add(1)
+
+		ctx, cancel := context.WithCancel(ctx)
+
+		// The composable system will continuously run, we are only interested in the first run on of the
+		// renderer to collect the variables we should stop the execution.
 		varsCallback := func(vv []*transpiler.Vars) {
 			varsArray = vv
 			wg.Done()
+			cancel()
 		}
 
 		ctrl, err := composable.New(log, cfg)