pbs: autogen key: rename old one if existing

pbs: autogen key: rename old one if existing

it could be debated do have some security implications and that
deletion is safer, but key deletion is a pretty hairy thing.

Should be documented, and people just should use delete instead of
autogen if they want to "destroy" a key.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>