ExecStart path in the systemd file is not correct
ExecStart path in the systemd file is not correct

In CentOS, the path of "ExecStart" in tacker.service file is
different from "/usr/local/bin/tacker-server", so it is unable
to start Tacker successfully.
In order to avoid this issue, fixed the code so that it is
available for multiple distributions.

Change-Id: I2e559523f6bcf57bffd693a4631f72ac339ac4eb
Closes-Bug: #1874384

openstack/tacker,stackforge/tacker,openstack/tacker,stackforge/tacker,openstack/tacker