Rename Pipeline to Sequence

[vaikas]

Problem
To reduce potential confusion with Tekton Pipelines, we should rename the Knative resource to Sequence which still conveys the meaning that the functions are executed sequentially.

Persona:
Developer

Exit Criteria
Once the resource has been renamed to Sequence.

Time Estimate (optional):
1-2

Additional context (optional)
https://github.com/tektoncd/pipeline

[grantr]

👍 OpenWhisk also calls these sequences: https://github.com/apache/incubator-openwhisk/blob/master/docs/actions.md#creating-action-sequences