[docs] document risk of combined ceph-osd

[Andrew Woodward]

We need to convert the following to documentation about not combing OSD's with other workloads:

It is not recommended to mix OSD / Compute role:

http://ceph.com/docs/master/rados/troubleshooting/troubleshooting-osd/#co-resident-processes

In general, under memory pressure it might even lead to deadlock. Here
is a good explanation from Sage Weil (the Ceph lead developer):

http://permalink.gmane.org/gmane.comp.file-systems.ceph.devel/12005

[Bogdan Dobrelya (bogdando)]

Note we have this already in docs as known issue, https://docs.mirantis.com/openstack/fuel/fuel-6.0/release-notes.html#placing-ceph-osd-on-controller-nodes-is-not-recommended. IIRC, there was a note about it in reference architecture or planning guide, but I cannot find it anymore. Perhaps, this warning should be provided in the planning guide as well