Ensure nova home dir is completely overridable

The var `nova_system_home_folder` was not used
consistently throughout the tasks. This can cause
deployment errors were a deployer to override the
var.

Change-Id: I359236041d2770de1a01b38dd8808c61d23faf26

tasks/nova_compute_key_populate.yml

@@ -16,7 +16,7 @@
 - name: Create the nova SSH config file
   copy:
     src: "ssh_config"
-    dest: "/var/lib/nova/.ssh/config"
+    dest: "{{ nova_system_home_folder }}/.ssh/config"
     owner: "{{ nova_system_user_name }}"
     group: "{{ nova_system_user_name }}"
     mode: "0644"