Allow creation of local configuration files for the server

Hi,

On the Stratum-0, it would be very helpful to be able to create extra files under 

/etc/cvmfs/repositories.d/<repo>/ 

If the files are consumed by alphabetic order, new ones can be used to override the defaults values in server.conf, for example. 

General idea is to avoid the need to update server.conf manually. Some Configuration Management Systems are not good modifying existing files, but they can always add new files. 

Also, in some sites, things like passwords, secrets, credentials... are stored on a protected host, and copied from it to the destination. This prevents the need to hardcode them in the Configuration Management System. In these cases, it would be helpful to be able to just copy the files, without editing server.conf

In general, manual configurations are hard to maintain. And having extra config files is the easiest way to deal with it.  

Is this something you would consider?

Cheers,

Jose