labs: remove autostart files in fast wbatch mode

In wbatch fast mode, scripts are copied to the autostart directory but
they are never executed and therefore not removed from there. This patch
fixes that.

Change-Id: I33f2881a0fbe8b91038825b25aa8bcd79074786e
Implements: blueprint openstack-training-labs
Merge "labs: remove autostart files in fast wbatch mode"