test/rocksdb: add rocksdb_commit_id file

This signals which RocksDB commit should be checked
out for the SPDK RocksDB tests.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: I1ba0be00747a2642b359b1e0e0c8c2c6d99cc4f1

Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/451772


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>