Option for "bosh-deploy" task to skip stemcell upload

[jochenehret]

What is this change about?

Add an option to the "bosh-deploy" task to skip the stemcell upload. This is useful in scenarios like the FIPS stemcell validation where the stemcell must be downloaded from a protected storage.

Please provide contextual information.

Will be used in FIPS stemcell validation pipeline: https://github.com/cloudfoundry/cf-deployment/pull/1135

Please check all that apply for this PR:

• [ ] introduces a new task
• [x] changes an existing task
• [ ] changes the Dockerfile
• [ ] introduces a breaking change (other users will need to make manual changes when this is released)

Change does not introduce regressions (i.e. default behaviour stays the same).

Did you update the README as appropriate for this change?

• [ ] YES
• [x] N/A

Docu is in task.yml.

How should this change be described in release notes?

Add option SKIP_STEMCELL_UPLOAD to the bosh-deploy task.

What is the level of urgency for publishing this change?

• [ ] Urgent - unblocks current or future work
• [x] Slightly Less than Urgent