The recent PR #883 replaced the standard overlay naming convention with
static names for each of the runtime and wwinit/system contexts; but the
method by which it left the "legacy" naming convention in place led to a
bug where warewulfd was still attempting to serve overlays with the
previous naming convention, though this name wasn't being built.
This PR refactors contextual overlay image naming to be, I think, a more
typical golang pattern, and updates affected code to match.
This PR also included tests for all functions modified by this change.
Fixes#896
Signed-off-by: Jonathon Anderson <janderson@ciq.com>
