Replace Job Semantics

[fnikolai]

Services and Cluster can be confusing.

Replacing it with the following semantics.

• Job: create a pod. No scheduling semantics.

• ParallelJob: create multiple concurrent Jobs. Scheduling semantics. From the workflow perspective, dependency can be built on the ParallelJob but not on the individual Job.

• SequentialJob: create a series of sequentially executed Jobs. No scheduling semantics. From the workflow perspective, dependency can be built on the SequentialJob but not on the individual Job.

Scheduling can be confusing.

Replace it with every, and distinguish it from until and assert.
(Although all these semantics use the same scheduling logic in the background).

• Questions: should we allow for concurrent state, metrics, and cron policies to be specified? If so, which operator is applicable ? Is there any case about this need?