update.yml doesn't offer to add additional variables for the upgrade script

[111andre111]

As of today the upgrade script has a limited hard coded amount of parameters:
https://github.com/elastic/ansible-elastic-cloud-enterprise/blob/master/tasks/ece-bootstrap/upgrade.yml#L3
It would be nice if the parameters could be customized in order to fulfill additional parameters that are documented here:
https://www.elastic.co/guide/en/cloud-enterprise/current/ece-installation-script-upgrade.html

Possibly a parameter can be introduce which later on can be used for customization.

[m-a-leclercq]

Hi andre, I have started integrating some of your requested changes here #150
I have a local branch that adds a few more of those, but it can get quite messy quite fast wit the jinja ifs so we might have to refactor some of those changes at some point.