VMware: handle case when VM snapshot delete fails

Minesweeper fails on occasion with concurrent access to files.

If a snapshot deletion fails with the backend exception
TaskInProgress then we will retry.

A generic decorator has been added to do the retry operations.

Closes-bug: #1310817

Change-Id: I8ed7a24ccd34aeea49352ac98f34ec2960edbf97
Merge "VMware: handle case when VM snapshot delete fails"