fix #1465: use a combobox for the crush rule instead of the id

fix #1465: use a combobox for the crush rule instead of the id

this patch does a few things

1. we introduce a new api call /nodes/nodename/ceph/rules
   which gets us a list of crush rules

2. we introduce a new CephRuleSelector which is a simple combobox
   with the data from the api call ceph/rules

3. we use this in the create pool window

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>