Add required dependencies to couchbase-server.service unit

There is a known issue for which Redhat has provided a fix at https://access.redhat.com/solutions/1751483. The fix requires to add dependencies (`local-fs.target` and `basic.target`) to `AFTER` and `REQUIRES` properties of the unit, that are currently missing in `couchbase-server.service` unit. If one thinks about it, it does make sense that `systemd` should wait for the local file system to complete booting up before starting Couchbase Server, so adding these dependencies should be fine. On executing the command `systemctl list-dependencies couchbase-server.service` we do see that `couchbase-server.service` has a dependency on `local-fs.target`. We should add these missing dependencies to `couchbase-server.service` unit.