Updated openstack/openstack
Project: openstack-dev/devstack  15ae2101bacd17a3809f868ff2e29a21c9243321

XenAPI: Fix race condition on tailing stack.log

Stack.log is created by stack.sh, which is started by run.sh.  Using
the pid from run.sh doesn't mean that stack.log has been created yet
so wait for the log to be created before tailing

Change-Id: I86e563478949cb7bbe0d1afd80b790ba70328e3a