Add configurable footer-message reports

Currently there is no way to add extra information to the end of a
report sent back by zuul. This adds a 'footer-message' field to the
pipelines allowing operators to leave extra information at the end
of a build result.

This is useful for leaving CI contact and debugging information.

Change-Id: Ieae62e845915fa3997353b6b425e215966a9338c
Document the Zuul triggers

Zuul send different sets of parameters depending on the event that
triggered a build, split the parameters in sub sub sections for some
clarity.

Added examples of ref-updated parameters.

Sub-sub section for the jenkins configuration.

Change-Id: I1edcd5c3a101e2afe0025ad4cd3e511582998999
Merge "Document the Zuul triggers"
Merge "Add configurable footer-message reports"