asynchronous SOCI index build

[tmokmss]

Builiding SOCI index takes decent time if a target contaier image is large, making iteration cycle slow. To avoid this, we allow to build an index asynchronously, returning SUCCESS response to CloudFormation immediately after CodeBuild job is started, and build an index after that.

Caveats: There is possibility that deployment successes while SOCI index fails to build. So we have to recommend the flag only for development/testing purposes.