Fix libvirt polkit settings

After the https://review.openstack.org/#/c/75314 merged
the /etc/polkit-1/rules.d/50-libvirt-stack.rules files contains
subject.user == '"stack"' instead of
subject.user == 'stack'.

Change-Id: I09f252b2d0e53f012facb9f7eaa21c1e1bdf492b
diff --git a/lib/nova_plugins/hypervisor-libvirt b/lib/nova_plugins/hypervisor-libvirt
index a550600..dc999ed 100644
--- a/lib/nova_plugins/hypervisor-libvirt
+++ b/lib/nova_plugins/hypervisor-libvirt
@@ -87,7 +87,7 @@
             cat <<EOF | sudo tee $rules_dir/50-libvirt-$STACK_USER.rules
 polkit.addRule(function(action, subject) {
     if (action.id == 'org.libvirt.unix.manage' &&
-        subject.user == '"$STACK_USER"') {
+        subject.user == '$STACK_USER') {
         return polkit.Result.YES;
     }
 });