XenAPI: Tail output from stack.sh

While installing devstack in XenServer's DomU we can easily
track the output so end-users have much more visibility of
both what is going on and what (if anything!) went wrong.

Change-Id: I7d12a13cea9dffda6c1388ce711b6adf76cae72c
Merge "XenAPI: Tail output from stack.sh"