Modify pci_passthrough hiera value as string

Hiera value of nova::compute::pci_passthrough should be a string.
It has been modified to JSON with the heira hook changes. Modifying
it again back to string.
Closes-Bug: #1675036

Change-Id: I441907ff313ecc5b7b4da562c6be195687fc6c76

puppet/services/nova-compute.yaml

@@ -52,7 +52,7 @@ parameters:
       For different formats, refer to the nova.conf documentation for
       pci_passthrough_whitelist configuration
     type: json
-    default: {}
+    default: ''
   NovaVcpuPinSet:
     description: >
       A list or range of physical CPU cores to reserve for virtual machine
@@ -101,7 +101,11 @@ outputs:
         map_merge:
           - get_attr: [NovaBase, role_data, config_settings]
           - nova::compute::libvirt::manage_libvirt_services: false
-            nova::compute::pci_passthrough: {get_param: NovaPCIPassthrough}
+            nova::compute::pci_passthrough:
+              str_replace:
+                template: "JSON_PARAM"
+                params:
+                  JSON_PARAM: {get_param: NovaPCIPassthrough}
             nova::compute::vcpu_pin_set: {get_param: NovaVcpuPinSet}
             nova::compute::reserved_host_memory: {get_param: NovaReservedHostMemory}
             # we manage migration in nova common puppet profile

releasenotes/notes/sriov-pci-passthrough-8f28719b889bdaf7.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - The ``pci_passthrough`` hiera value should be passed as a string
+    (`bug 1675036 <https://bugs.launchpad.net/tripleo/+bug/1675036>`__).