Adjust documentation and behaviour of cpuset

1. Detail the caveats and specific situations and ref the documentation
which will provide more details.

2. Always install the configs, but use /etc/default/ceph-osd-cpuset to
control if the script does anything or not (so, the "osd" cset set is
always active just not set in a special way.

roles/pvc/templates/ceph/ceph-osd-cpuset.service.j2

@@ -1,6 +1,5 @@
 # PVC Ceph OSD cpuset service unit
 # {{ ansible_managed }}
-{% set cset_host = pvc_shield_osds_cset | selectattr('hostname', 'equalto', inventory_hostname) %}
 [Unit]
 Description = Ceph OSD cpuset shield creation
 Before = ceph-osd@.service libvirtd.service