Refactor launcher to merge meta from parent event by default

[VonnyJap]

What happened:

When an event is triggered by remote pipeline, the remote build is used as the parentBuildId in the subjected build. When launcher is started, the merging of metadata occurs. During a restart case, sometimes the metadata from parentEvent is expected but with the current implementation, the metadata from parentBuildId wins over those from parentEvent, hence we will end up in a situation where some metadata are missing.

See https://github.com/screwdriver-cd/launcher/blob/8e928b64cc055140b6694bcf79a1de673f90ebfc/launch.go#L485-L514

What you expected to happen:

Metadata from parentEvent should be merged by default.

How to reproduce it:

Setup a pipeline with a remote build trigger and try to set metadata during the run of a build in a particular event, try to do a restart, it will be observed that metadata is missing.