systemd was updated in RHEL 7.2 and thus also in CentOS 7.2. This update breaks compatibility with how ambari-server starts up. There is a simple fix, just create the file /usr/lib/systemd/system/ambari-server.service with these contents:
There's probably a better way to work with systemd in a more systemd-ish way, but this restores compatibility for now with minimal effort. We should update the RPM to install this file if the system is managed by systemd.