Libvirt:Instance resize confirm issue against NFS

when I test resize-confirm on NFS (KVM) using at least two
compute nodes, OSError will occur when try to cleanup the
backup instance directory and will make instance status
"Error".

One way to cleanup resize is using delay_on_retry and
maximum attempts setting. Replace rmtree() because
execute() have more parameters choices.

Closes-Bug: #1248019

Change-Id: Ifb9a500bbba805af0317307c2e7d6903dcd02ad1
(cherry picked from commit 17d131661740e452e9729d9ac8881e6fede23dd7)
Merge "Libvirt:Instance resize confirm issue against NFS" into stable/havana