Improve Ceph documentation

[vickmp]

Description Some sections should be updated adding more info and context for more Ceph uses cases:

• Add a description on how to add disks devices to the inventory and dedicated devices as well.
• Add a warning comment to wiping out all the disks added to the inventory prior the execution of the one-deploy tool. Some advises here.
• Add a Troubleshooting/Basic Operations section with information on how to remove an OSD and how to create additional pools (with potentially different replicas, PGS, etc...) after the installation.

Progress Status

• [ ] Code committed
• [ ] Testing - QA
• [ ] Documentation (Release notes - resolved issues, compatibility, known issues)

[rsmontero]

https://github.com/OpenNebula/one-deploy/wiki/arch_single_ceph