[CI] Evaluate jobs parallelization

[jmaupetit]

Purpose

Since the CI refactoring in #112, one must add a new job for each new release. This is great to run build in parallel, but we should test the CI-builtin parallelization mechanisms [1].

[1] https://circleci.com/docs/2.0/parallelism-faster-jobs/#using-environment-variables-to-split-tests

Proposal

• Define a generic build job with parallelism: 4 (or any other relevant value)
• Use the circleci tests split command to split releases and build one release per container