Default bootstrap_server_id

When blacklisting all servers from the primary role, the yaql expression
to get the bootstrap_server_id value fails as it tries to index the list
at the 0'th element. In this case, default the bootstrap_server_id value
to a constant string which won't match any actual server id's.

Change-Id: Ibb26245156675f64709bab075875ce4b498b4db6
Closes-Bug: #1785665

common/deploy-steps.j2

@@ -269,7 +269,11 @@ resources:
     properties:
       value:
         yaql:
-          expression: $.data.items().orderBy($[0]).first()[1]
+          # Use a constant string of "bootstrap_server_id" when there are no
+          # servers in the primary role, such as in the case when all
+          # Controllers are blacklisted. No server id's will match the string
+          # which is what we want when all are blacklisted.
+          expression: switch($.data = {} => "no_bootstrap_server", $.data != {} => $.data.items().orderBy($[0]).first()[1])
           data: {get_param: [servers, {{primary_role_name}}]}
 
 # Artifacts config and HostPrepConfig is done on all roles, not only

releasenotes/notes/default-bootstrap-server-id-eff49ef40bfde414.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - Previously, when blacklisting all servers of the primary role, the stack
+    would fail since the bootstrap server id was empty. The value is now
+    defaulted in case all primary role servers are blacklisted.