Tidy and clarify create-sandbox-instance.yml

[mrenvoize]

• We always want to wait for 1 minute whilst koha-testing-docker initialises so remote the 'when'.
• We always want to add the docker instance to the hosts, but only need to do it once.
• We optionally want to checkout a branch after the docker start
• We optionally want to apply bugs to our branch after the docker start