Add a `releaseAsLatest` param to release pipeline
Add a `releaseAsLatest` param to release pipeline

This PR adds a `releaseAsLatest` param to the pipeline and task which
defaults to true.  When overridden and set to "false" the pipeline will
not tag any images as ":latest" or upload to the /latest directory of
the releases bucket.

The use case for this is to for release candidates and patching older
releases.

Based on tektoncd/pipeline#2242

Signed-off-by: Dibyo Mukherjee <7d445be0c0af41c00c371a1f7b3669c3331901f5@google.com>

tektoncd/triggers,tektoncd/triggers