FDB Should not start on install when cluster file is detected

https://github.com/apple/foundationdb/blob/85d0dec585b276e3588dfbe6b1c25c07f38a417e/packaging/deb/DEBIAN-foundationdb-server/postinst#L36

It would be preferred to have FDB not start on install when the system detects a pre-existing fdb.cluster or foundationdb.conf file. We often find ourselves adding dozens of processes and/or multiple machines which can be quite disruptive to the cluster depending on current workload.

We are currently maintaining our own version of the debian postinst which skips the start step. However it gets problematic maintaining our own pipeline to change a single file in the deb. We'd like to suggest that FDB should either not start on installation or only start when it detects `$NEWDB == 1`

Start with packaging/deb/DEBIAN-foundationdb-server/postinst at the linked installation step, and inspect how the existing fdb.cluster, foundationdb.conf, and $NEWDB conditions are handled. Done means installation preserves an existing cluster without starting FDB, while still handling a new database according to the selected condition.

Written by the indexing model from the issue text.