after installing mongodb-server-2.6.12-5.el7.x86_64, I cannot start mongod. It works if I revert to mongodb-server-2.6.12-4.el7.x86_64. Details as follows:

Job for mongod.service failed because the control process exited with error code. See "systemctl status mongod.service" and "journalctl -xe" for details.

Running Systemctl status gives me:
Jan 29 22:41:35 localhost.localdomain systemd[1]: Starting High-performance, schema-free document-oriented database...
Jan 29 22:41:35 localhost.localdomain mongod[1313]: No map found at top level of YAML config
Jan 29 22:41:35 localhost.localdomain mongod[1313]: try '/usr/bin/mongod --help' for more information
Jan 29 22:41:35 localhost.localdomain systemd[1]: mongod.service: control process exited, code=exited status=2
Jan 29 22:41:35 localhost.localdomain systemd[1]: Failed to start High-performance, schema-free document-oriented database.
Jan 29 22:41:35 localhost.localdomain systemd[1]: Unit mongod.service entered failed state.
Jan 29 22:41:35 localhost.localdomain systemd[1]: mongod.service failed.

The mongod.conf is still in the old format. Switching the conf file to yaml does not help:

Jan 29 22:27:36 localhost.localdomain systemd[1]: Starting High-performance, schema-free document-oriented database...
Jan 29 22:27:36 localhost.localdomain mongod[1298]: No map found at top level of YAML config
Jan 29 22:27:36 localhost.localdomain mongod[1298]: try '/usr/bin/mongod --help' for more information
Jan 29 22:27:36 localhost.localdomain systemd[1]: mongod.service: control process exited, code=exited status=2
Jan 29 22:27:36 localhost.localdomain systemd[1]: Failed to start High-performance, schema-free document-oriented database.
Jan 29 22:27:36 localhost.localdomain systemd[1]: Unit mongod.service entered failed state.
Jan 29 22:27:36 localhost.localdomain systemd[1]: mongod.service failed.