Update stacki frontend install docs for 5.6

[bsanders]

Note stacki's wiki is maintained in a separate repo! https://github.com/Teradata/stacki-wiki/

frontend install.py now ships as an rpm in the stacki iso. Use that instead of fetching the one linked (from master!) by mounting the iso, installing the foundation-python3 and stacki-fab rpms, and proceed as before.

The doc is here:

https://github.com/Teradata/stacki/wiki/Frontend-Install-Existing

The doc in the wiki repo is here:
https://github.com/Teradata/stacki-wiki/blob/master/wiki/Installation/Frontend-Install-Existing.md