fix: add default ROLLOUT_RESOURCE_TYPE=deployment

Needing to specify ROLLOUT_RESOURCE_TYPE is an obscure implementation detail that most user should not need to care about. We don't even have built-in support for other types, so we should not force users to set it.

This is especially relevant to users who include Jobs/Deploy.gitlab-ci.yml instead of Auto-DevOps.gitlab-ci.yml.