Change value for pci_alias parameter when empty

https://review.openstack.org/#/c/492172/ has changed how
pci_alias is managed in puppet-nova module and false is
not longer a valid value.

This patch changes the value for pci_alias when it's not
set in packstack to the default value, $::os_service_default

Change-Id: I663511ec2cf8d16461c75484bfc5481c45558f6a

packstack/puppet/modules/packstack/manifests/nova/api.pp

@@ -25,7 +25,7 @@ class packstack::nova::api ()
     }
 
     if hiera('CONFIG_NOVA_PCI_ALIAS') == '' {
-      $pci_alias = false
+      $pci_alias = $::os_service_default
     } else {
       $pci_alias = hiera('CONFIG_NOVA_PCI_ALIAS')
     }