pki: Cleanup CA creation

Remove old certs on CA creation
Example scenario where this is required:
engine-setup ran with websocket-proxy enabled
engine-cleanup
engine-setup ran with allinone enabled

Without this change, the cert creation for the vdsm host in the second
run will fail, because it runs as user ovirt, and the previous
engine-setup left certs/1004.pem owned by root, so can't be rewritten.

Also, for completeness, remove old keys and requests, as well as other
files, add error checking to backup/remove, etc.

And fix minor error.

Change-Id: Ibab2884b461da0a723ea9d2040985a2429af1632
Signed-off-by: Yedidyah Bar David <didi@redhat.com>
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>