[Coverage] Use PR# as GCS file name & changed ACL (#522)
[Coverage] Use PR# as GCS file name & changed ACL (#522)

* [Coverage] Use PR# as GCS file name & changed ACL

1. use PR number as object name to enable indexing. Indexing allows
incremental coverage done on a commit #514

2. make the object project-private. Without specifying, object will be
public (which is strange because GCS document says the default is
project-private) #513

3. changed the name SCRIPT_ROOT to ELAFROS_ROOT, because the name SCRIPT_ROOT may mislead the reader to thinks it's the directory of the script. #503

* added a folder to hold all the commit # based profiles to keep the original folder cleaner

knative/serving,knative/serving