CBLT-179:  instantiate PRODUCT_VERSION, COMMIT_SHA in api.go

See sync_gateway commit:  1b86eebeb7805213b566007197776362c4d768a0

Change-Id: I2d73ba85c111988d1ddab3167045bc74c5a94faf
Reviewed-on: http://review.couchbase.org/35683
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>