Fix not found being raised when iterating

Instead of re-raising a not found exception just
silence those exceptions when iterating (and do not
yield that job back in the first place) and then
remove the job from the jobboard internals to avoid
further iteration.

Fixes bug 1304562

Change-Id: Ibc6d3ea55dc19e70104bfc801283b8ad919496c4
Doc adjustments

- Cleanup some grammar and adjust some wording usage.
- Add more docs to utils about what should and should
  not be used.
- Add more engine docs about how each one is used and
  tips and notes about each.
- Line length adjustments (might as well keep it somewhat
  in the normal range of what openstack code expects)

Change-Id: Ice6711f00e2b50e0bee777388c0555d79cc6e1b0
Merge "Fix not found being raised when iterating"
Merge "Doc adjustments"