nova: Remove versioned default machine types

Instead rely on the unversioned defaults provided by Nova [1].

The use of versioned machine types should be limited to specific
downstream releases to ensure the corner case of live migrations between
X.Y+1 and X.Y releases of RHEL will work.

Removal of these versioned machine types also allows TripleO to test
with both EL based and Fedora based compute hosts that version their
machine types differently as discussed previously in bug #1819452.

[1] 50fdbc752a/nova/virt/libvirt/utils.py (L566-L575)

Change-Id: I808c033c34dfe1068ebe17dc72fdee5ef63613d8
Related-Bug: #1819452

deployment/nova/nova-compute-container-puppet.yaml

@@ -415,7 +415,7 @@ parameters:
   NovaHWMachineType:
     description: >
       To specify a default machine type per host architecture.
-    default: 'x86_64=pc-i440fx-rhel7.6.0,aarch64=virt-rhel7.6.0,ppc64=pseries-rhel7.6.0,ppc64le=pseries-rhel7.6.0'
+    default: ''
     type: string
     tags:
       - role_specific

releasenotes/notes/nova-machine-type-default-changed-27244a925f6d6200.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    The ``NovaHWMachineType`` parameter now defaults to an empty string with
+    per architecture machine type defaults instead being provided directly by
+    OpenStack Nova.