Move Cluster-file to Data Directory

#3864 points out a problem with the way we currently use cluster-files in FDB. Changing files in `/etc` is not good style. Instead we should write the cluster-file into the data directory. The way this can be implemented is as follows:

1. By default, look for the cluster file in `${DATA_DIR}/fdb.cluster` (in `fdbserver` - for the client we should probably remove `/etc/foundation/fdb.cluster` from the search list and only look in the current working dir or let the user configure it through an option).
1. Set the default value of `--seed_cluster_file` to `/etc/foundation/fdb.cluster`

Alternatively we could also use something like `/var/lib/foundationdb` for the cluster file path. However, having a global cluster file is probably not a good idea (one could in theory run multiple processes on a machine that participate in different clusters - also the data directory is the only directory we know FDB will have write access to).

Start in the fdbserver cluster-file lookup and the client’s cluster-file search and configuration handling. Trace the default paths and the --seed_cluster_file option, then verify that fdbserver uses ${DATA_DIR}/fdb.cluster and that client lookup no longer depends on /etc/foundation/fdb.cluster; resolve the alternative path choice before implementation.

Written by the indexing model from the issue text.