Add idempotent droplet creation

[kathap]

Add idempotent droplet creation:

• if there exists already a droplet, take that one and ensure buildpack_lifecycle_data is attached (but don’t overwrite)
• if not create a new droplet

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• A short explanation of the proposed change:
  Add idempotent droplet creation. This will add one simple SQL query to check for already existing droplet:
  SELECT * FROM "droplets" WHERE "build_guid" = 'abcd-1234' LIMIT 1;

• An explanation of the use cases your change solves
  Fix hanging staging on cc-uploader kill caused by 2 created droplets with same build_guid. The Problem was if cc-uploader would run into uplaod timeout or cc-iuploader would be killed hard, then diego sees its upload request failed, diego sends a new upload reuqest to a healthy VM and with that a new droplet was created for the same build, which led to staging saving error "diego.staging.buildpack.saving-staging-result-failed", "error":"undefined method `buildpacks' for nil:NilClass","build" because completion handler tried to save the result with the first droplet guid but that droplet had no longer buildpack_lifecycle_data assigned, those were now assigned to the 2. droplet. Build and droplets remained in state STAGING forever and staging finally failed because of timeout.

• Links to any other associated PRs

• I have reviewed the contributing guide

• I have viewed, signed, and submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests

[jochenehret]

Looks good, just one question regarding logging.