Remove stale VMware snapshot alerts when updating or deleting VMware host

The middleware alert plugin 'vmware_snapshot.py' sends emails in 2 scenarios.

1. when taking a vmware aware snapshot fails
2. when deleting a vmware aware snapshot fails

It looks for 2 sentinel files and sends an email alert accordingly.

The autosnap.py script is the script responsible for creating these files, but there is a situation where if the customer was once using vmware aware snapshots, and then removes them, the sentinel files are not being removed so the customer continually gets an alert stating their snapshots of the vm failed.