[bradm] Make instances-path have no default, and update code to reflect this

config.yaml

@@ -84,7 +84,6 @@ options:
     type: string
     description: Network interface on which to build bridge
   instances-path:
-    default: ''
     type: string
     description: Instance path to use - empty means default of /var/lib/nova/instances
   config-flags:

hooks/nova_compute_context.py

@@ -99,7 +99,7 @@ class NovaComputeLibvirtContext(context.OSContextGenerator):
             # nova.conf
             ctxt['live_migration_uri'] = 'qemu+ssh://%s/system'
 
-        if config('instances-path') != '':
+        if config('instances-path') is not None:
             ctxt['instances_path'] = config('instances-path')
 
         return ctxt

hooks/nova_compute_hooks.py

@@ -84,7 +84,7 @@ def config_changed():
     else:
         disable_shell(user='nova')
 
-    if config('instances-path') != '':
+    if config('instances-path') is None:
         fp = config('instances-path')
         fix_path_ownership(fp, user='nova')