Bind secrets to their playbooks

Secrets are proving less useful than originally hoped because they
can not be effectively used in any jobs with untrusted children.

This change binds the secrets to the playbooks which use them, so
that child jobs are unable to access the secrets.  This allows us
to create jobs with pre/post playbooks which use secrets which
are suitable for other jobs to inherit from.

Change-Id: I67dd12563f3abd242d6356675afed1de0cb144cf
diff --git a/doc/source/user/jobs.rst b/doc/source/user/jobs.rst
index 7f1c3cb..5f36c30 100644
--- a/doc/source/user/jobs.rst
+++ b/doc/source/user/jobs.rst
@@ -121,6 +121,9 @@
 
  {{ credentials.username }} {{ credentials.password }}
 
+Secrets are only available to playbooks associated with the job
+definition which uses the secret; they are not available to playbooks
+associated with child jobs or job variants.
 
 Zuul Variables
 ~~~~~~~~~~~~~~