fix #1430: ceph init: allow to specify separate cluster network

fix #1430: ceph init: allow to specify separate cluster network

Allow to specify a separate cluster network when initializing ceph.
Ceph docs[0] imply a possibility for performance increase and
enhanced security in environments where the public network serves not
fully trusted peers, which could else provoke a DOS to the cluster
traffic[0].

Make this optional, but if passed `network` is required too.

[0]: http://docs.ceph.com/docs/luminous/rados/configuration/network-config-ref/

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>