Initialize log directory required by waagent

waagent assumes that the `/var/log/azure` directory exists and raises an
exception in the telemetry module if it does not.

```sh
ERROR TelemetryEventsCollector ExtHandler Event: name=WALinuxAgent,
op=ExtensionTelemetryEventProcessing, message=Unknown error occurred
when trying to collect extension events:[Errno 2] No such file or
directory: '/var/log/azure'
```

Author: s4heid

stemcell_builder/stages/system_azure_init/apply.sh

@@ -33,6 +33,7 @@ run_in_chroot $chroot "
   sudo rm -fr WALinuxAgent-${wala_release}
   rm wala.tar.gz
 "
+mkdir -p $chroot/var/log/azure
 cp -f $dir/assets/etc/waagent/waagent.conf $chroot/etc/waagent.conf
 cp -f $dir/assets/etc/waagent/walinuxagent.service $chroot/lib/systemd/system/walinuxagent.service
 chmod 0644 $chroot/lib/systemd/system/walinuxagent.service