Sane handling of ccache storage

[TheJJ]

We set up ccache for openage (SFTtech/openage#1000), which requires a persistent storage device for each VM.

This implies that this storage device must not be mounted twice at once. Which is only possible if each VM can only run alone at a given moment.

Enabling such a "limit" is a missing feature in falk and kevin. It should be configured in the falk.conf for each machine.

[TheJJ]

Better solution: Use NFS instead of the persistent storage device. Then this feature is obsolete and the file system can't corrupt.

Parallel access has to be handled anyway by applications like ccache.