Summary:
first, the config `platform.additional` now is correctly used
when generating YBA config files.

Second, as_root config now has a sane default during install. This
handles the case where an existing config file is used for a new
install and does not contain the as_root entry. It defaults to
the user that ran the install command.

Test Plan:
install test without as_root in the config
upgrade test

Reviewers: muthu

Reviewed By: muthu

Differential Revision: https://phorge.dev.yugabyte.com/D43882