Retry to create snapshot and clone after failure

New config value: snapshotDelayMin is introduced to retry with the
same action when next task schedule comes, withing the
snapshotDelayMin defined time delay

This mechanism is for the consideration of multiple snapshot/clone
action on the same vm, when only one action can succeed at a time,
so other actions can retry it at later time.

This patch fiexs #6

Signed-off-by: walteryang <walteryang47@gmail.com>