api: ceph: add endpoint to fetch config keys

api: ceph: add endpoint to fetch config keys

This new endpoint allows to get the values of config keys that are
either set in the config db or the ceph.conf file.

Values that are set in the ceph.conf file have priority over values set
in the conifg db via 'ceph config set'.

Expects the --config-keys parameter as a semicolon separated list of
"<section>:<config key>" where the section is a section in the ceph.conf
or config db. For example: global:osd_pool_default_size

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>