Fix experimental trove-image-build gate job

Fixed the experimental trove-image-build gate job to:
- Use /opt/trove-image-build instead of /opt/stack to avoid confusion
  with dvm-gate jobs.
- Export the right value for KEY_DIR
- Copy the built image to the correct location on tarballs.o.o

Change-Id: Idc1ea86fe7c692f318d5c6b004f73c10293d4607
Merge "Fix experimental trove-image-build gate job"