Fix starting mongodb under RHEL/CentOS 7 with systemctl

As noted in #330, the cookbook currently fails to start under CentOS 7. This pull request should fix that. Here's a description of the changes:

- The main issue was that systemctl seems to pay particular attention to the `pidfile` comment that was in the header of the init script. Since a pidfile wasn't actually being written, this was causing systemctl to wait indefinitely thinking the process never started. I removed the `pidfile` reference in the init script, and that allows the service to start.
- When init scripts are changed on disk after they're installed, systemctl wants you to run `systemctl daemon-reload`. This action is now performed anytime the init script is modified, but only on RHEL 7+ systems.
- I also updated the `config` comment in the init script to reference the correct config file. I'm not sure if systemctl cares as much about this setting, but I noticed it was incorrect (it referenced mongod.conf instead of mongodb.conf), so given the pidfile comment issues, I went ahead and updated that to be correct.