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
Merge "Libvirt:Instance resize confirm issue against NFS"