add log if droplet already exists

Author: kathap

app/actions/droplet_create.rb

@@ -53,6 +53,7 @@ def find_or_create_buildpack_droplet(build)
       DropletModel.db.transaction do
         existing = DropletModel.where(build_guid: build.guid).first
         if existing
+          Steno.logger('droplet_existing').info("existing droplet found: #{existing.guid}")
           if build.cnb_lifecycle?
             existing.cnb_lifecycle_data ||= build.cnb_lifecycle_data
           else