Build: Adjust build chain for Travis/SauceLabs

- Minor description updates and better differentiate internal tasks
- Make the SauceLabs an self contained external task
- Clean up assemble task and add "tests" target for SauceLabs/Mocha
runner
Merge pull request #3197 from nschonni/v4-travis-sauce-build-cleanup

Build: Adjust build chain for Travis/SauceLabs