1. Find out what package installed the config file:

$ dpkg -S unity-greeter.conf
  unity-greeter: /etc/lightdm/unity-greeter.conf

As you can see, the name of the package is unity-greeter.

2. Rename (or delete) the config file you wish to restore:

sudo mv -i /etc/lightdm/unity-greeter.conf /etc/lightdm/unity-greeter.conf.bak

3. Run the following command, replacing <package-name> with the name of the package:

sudo apt-get -o Dpkg::Options::="--force-confmiss" install --reinstall <package-name>

4. If everything worked as expected, you should get this message:

Configuration file `/etc/lightdm/unity-greeter.conf', does not exist on system. Installing new config file as you requested.

A Practical example when needing to reinstall all of the PulseAudio configuration files:

apt-cache pkgnames pulse |xargs -n 1 apt-get -o Dpkg::Options::="--force-confmiss" install --reinstall