Avoid the call to images index.yml when the wlp binary is already available

[leochr]

Avoid the call to https://public.dhe.ibm.com/ibmdl/export/pub/software/websphere/wasdev/downloads/wlp/index.yml (for example) when the wlp.zip is already downloaded.

Calls to DHE timeouts sometimes, so better to avoid unnecessary calls

[idlewis]

@leochr Both these PRs are ready for review, which I believe is all the work needed for this issue.