install.sh and existing snaptool.yml

[bruce-weka]

on new installs, NOT overwriting an existing snaptool.yml gets confusing if a first install attempt fails due to the snaptool.yml contents. Subsequent attempts at running install.sh will just preserve and re-use the bad yml file. Options:
-- better messaging about re-using the /opt/weka/snaptool file
-- ask about preserving vs overwriting snaptool.yml if it exists in /opt/weka/snaptool
-- rm/rename /opt/weka/snaptool.yml if the installer didn't find one to begin with, and the install fails?

[bruce-weka]

more options
-- separate install vs upgrade scripts. The issue exists really because install.sh tries to do upgrades, too.
-- "--clean" or similar option for install.sh that cleans up previous install files in install destination