Fix unstack.sh to stop stop_dstat when it is disabled.
Fix unstack.sh to stop stop_dstat when it is disabled.

unstack.sh always stop dstat process, however unstack.sh show 'dstat: no process
found' when dstat is disabled.
This patch stop function of stop_dstat, when dstat is disabled.

Backported from https://review.openstack.org/#/c/166709/

Change-Id: If9054826bed8a7fedd4f77ef4efef2c0ccd7f16e
Closes-Bug: #1435148

tianweizhang/devstack,tianweizhang/devstack,eshnil2000/devstack