Default to starting two Elasticsearch nodes instead of one in "stack up"

[jsvd]

stack up is extremely useful to manually smoke test the stack, but one thing I miss is sometimes having two nodes instead of one.
This is because some features only show their true potential on multi node setups and also rendering some UI components for a single node isn't enough.

Adding a second node would require tweaking the docker compose template and correctly setting the tls configuration.

[jsoriano]

We have to keep in mind that elastic-package happens to be able to manage stacks, but at the end it is intended for package development. In general one node is enough there.

So I wouldn't default to two nodes, but I agree that it can be useful in some cases and we can definitely add an optional setting to start more nodes.