Use senlin resources to manage VDU auto-scaling function

Tacker already supported VDU auto-scaling function by using Heat auto-scaling group, but Heat is not powerful enough to recover if auto-scaling failed(Heat can only use 'stack-update' to update the stack ). Senlin is a clustering service which can provide strong ability for auto-scaling function, it can handle the problems which are difficult to Heat. So invite Senlin resource to Tacker as another option to VDU auto-scaling feature.