Update flag day document

Update flag day document

Prevent false -1 from api-crc job.
Add more details on CSIT side of things.
Minimize the breakage of trending jobs.

Also: Add motivation section explaining
why earlier solutions are not good enough.

- Needs more polishing.
- VPP/CSIT, Committer/Developer can be defined in a separate section.
- VPP/CSIT trending and verify jobs can be defined in a separate section.
- The main process section should list only steps.
 - Comments on which jobs are broken/fixed can be moved to a separate section.
 - Name phases, so progress can be described better than "step 9".
- Emphasize that only one API change can be in the process.
  All other changes need to be rebased (to clear any previous +1)
  before next cycle starts.

Change-Id: Ie64a20db3f4abad3c8cddbf13705ff006e5a6382
Signed-off-by: Vratko Polak <vrpolak@cisco.com>