Add max-nodes-per-job tenant setting

This change adds a tenant setting to limit the amount of nodes a change
can request.

Change-Id: I8bac33cbb3a0438375462fed8124cc150649267f
diff --git a/doc/source/admin/tenants.rst b/doc/source/admin/tenants.rst
index 18ec381..1f8f7db 100644
--- a/doc/source/admin/tenants.rst
+++ b/doc/source/admin/tenants.rst
@@ -28,6 +28,7 @@
 
   - tenant:
       name: my-tenant
+      max-nodes-per-job: 5
       source:
         gerrit:
           config-projects:
@@ -48,6 +49,10 @@
   characters (ASCII letters, numbers, hyphen and underscore) and you
   should avoid changing it unless necessary.
 
+**max-nodes-per-job** (optional)
+  The maximum number of nodes a job can request, default to 5.
+  A '-1' value removes the limit.
+
 **source** (required)
   A dictionary of sources to consult for projects.  A tenant may
   contain projects from multiple sources; each of those sources must