chore(e2e): Clean up comments in enos.vars

[moduli]

This PR makes some updates to enos.vars.hcl to clarify comments and make it easier to onboard.

Some additional changes were included to accommodate these changes

• enos-local.vars.hcl was added to .gitignore to allow a local copy of variables without potentially committing them
• Some variables were made optional so that users who don't need to run e2e tests can ignore them.

https://hashicorp.atlassian.net/browse/ICU-14090