Clean up cloud-init artifacts

Use the cloud-init clean command instead of removing the files by hand (/var/lib/cloud)

Ref. https://cloudinit.readthedocs.io/en/latest/topics/cli.html#clean
```
$ sudo cloud-init clean -h
usage: /usr/bin/cloud-init clean [-h] [-l] [-r] [-s]

optional arguments:
  -h, --help    show this help message and exit
  -l, --logs    Remove cloud-init logs.
  -r, --reboot  Reboot system after logs are cleaned so cloud-init re-runs.
  -s, --seed    Remove cloud-init seed directory /var/lib/cloud/seed.
```

Signed-off-by: Luca Carrogu <[email protected]>

Author: lukeseawalker

cookbooks/aws-parallelcluster-install/files/default/base/ami_cleanup.sh

@@ -1,8 +1,9 @@
 #!/bin/bash
 
+# clean up cloud init artifacts https://cloudinit.readthedocs.io/en/latest/topics/cli.html#clean
+cloud-init clean -s
+
 rm -rf /var/tmp/* /tmp/*
-rm -rf /var/lib/cloud/instances/*
-rm -f /var/lib/cloud/instance
 rm -rf /etc/ssh/ssh_host_*
 rm -f /etc/udev/rules.d/70-persistent-net.rules
 grep -l "Created by cloud-init on instance boot automatically" /etc/sysconfig/network-scripts/ifcfg-* | xargs rm -f