ci/cd pipeline support

[zxkane]

closes #11

Add below capabilities,

• implement across accounts to deploy different stages of opentuna. Having below workflow,
  commints on master branch -> build & test the code -> deploy to stage uat in account A -> manual approval -> deploy to stage ‘prod’ in account B
• chatops support, approve/reject the prod deployment via slack
• update doc
• add unit tests

Known limitations,

• the pipeline does not support automatically update itself
• the CD always uses latest commit of the branch. step functions and codebuild startbuild don’t support override source commit