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
Raise HTTPNotFound error from V2 cert show API

There might be possibility of not having the CA file while
requesting certificate show API. API should raise HTTPNotFound
for clear error.
This patch raise this exception for V2 API. This has been fixed in
V3 API.

Adding UT also for the same.

Change-Id: Iafb33fce6433a31f9767067bbc45b95acd4ee511
Merge "Raise HTTPNotFound error from V2 cert show API"
Merge "VMware: handle case when VM snapshot delete fails"