More explanation for tree mode: fanout, -o

[degremont]

Tree mode introduces a lot of "blurry" behaviour and it is difficult for users to understand what is the current behaviour for a couple of options. This should be clarify and documented:

Could you tell me easily, what is the behaviour, in tree mode of this option:

• --option? Is this for gateway, for final connections to the nodes, both?
• fanout? global or per gateway? If you hit Enter during a clush command, ouput is ... strange and does not help users understand what is the current behaviour.

I assume there are more of these.

Regarding fanout, clush --fanout=1 is broken...

[degremont]

• fanout=1 issue will be solved by another ticket.
• For now, I think we should just add explanation in documentation and man pages to simply explain that fanout value (from config or overriden by CLI option) will be copied on every gateway. With an example. This behaviour should be modified in the future, but for now, let just explain what it currently does.
• The strange output should be verified with the fanout fix to verify things are clearer.

And that should be enough for this ticket.

[thiell]

• fanout=1 has been fixed in fanout=1 with gateways is broken + per worker fanout #322
• -o or -O options propagation needs a fix (see https://review.gerrithub.io/#/c/298393/)
• fd_max is a specific case that could be handled in 1.8 (see fd_max is only supported on head node and by clush #325)
• options and fanout in tree mode documentation improvements @ https://review.gerrithub.io/#/c/298394/