fix #2515: ui: ceph pool create: use configured defaults for size and min_size

fix #2515: ui: ceph pool create: use configured defaults for size and min_size

Instead of hard coded defaults for the size and min_size parameter,
check if we have defaults configured in the ceph.conf or config db and
use those.

There are clusters where different defaults are needed. For example if
the cluster spans two rooms and needs to survive the loss of one. A
size/min_size of 4/2 are common defaults in such a situation.

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>
Tested-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>