Default values should be carefully selected

[mlajkim]

Background

Default value like DELETE_INSTANCE_ID=true is wrong as it requires athenz permission and outputting error message without permission isn't the best

TODOs

- [ ] Check the default values
- [ ] Check if every env must be modifiable under Makefile

[WindzCUHK]

Deleting the instance certificate should be the default behavior because of the following:

• improve security
• reduce ZTS load

I think the focus should be the error message.