weldr/upload/gcp: make Object optional

Previously, it was expected from the user to provide the Object name
when uploading image to GCP. The object name does not matter much,
because the object is deleted once image import finishes. Make
the specification of the object name optional and generate it if not
provided.

Adjust the GCP Weldr test case to not provide the Object name when
uploading the image.

The user can still provide the Object name if needed.

test/cases/gcp.sh

@@ -201,7 +201,6 @@ provider = "gcp"
 [settings]
 bucket = "${GCP_BUCKET}"
 region = "${GCP_REGION}"
-object = "${GCP_IMAGE_NAME}"
 credentials = "$(base64 -w 0 "${GOOGLE_APPLICATION_CREDENTIALS}")"
 EOF