Update existing examples for both local and remote GreptimeDB

[sunng87]

We can use environment vairables to control the demos to use local GreptimeDB OSS or remote GreptimeCloud instances as introduced in #51 . We can further update existing demos to adopt this.

The approach is:

• use environment variables with default value for GreptimeDB endpoint in configuration files
• provide default greptime.env.sample for use to copy and fill
• use envsubst docker image as a init container as in feat: alloy and greptimecloud in demo #51 to check and substitute variables