Currently playbook slurps constraints.txt valid for a first host in a
batch and then uses it for each other host.
It saves time but batch may consists of hosts with different operating
systems or even OSA versions and current mechanism doesn't take this
into account.
In my opinion we should slurp constraints.txt separately for each host.
Change-Id: I9ba1d342af13e9c923151f735940bda29bf1b262