Split overlays into discrete functionality and test

- Closes #987

To make the overlays testable in isolation, and after discussion with
the TSC, removed host/time/source from overlay files.

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

internal/pkg/overlay/overlay.go

@@ -26,7 +26,7 @@ var (
 )
 
 /*
-Build all overlays (runtime and generic) for a node
+Build all overlays for a node
 */
 func BuildAllOverlays(nodes []node.NodeInfo) error {
 	for _, n := range nodes {