add on_add and on_delete hooks

add on_add and on_delete hooks

on_add_hook allows to encapsulate storage specific add steps, like
copying a keyring (RDB) or creating a volume group (LVM) in a clean
manner.
The same for deletion with on_delete_hook, here all should be cleaned
up, as much as possible.

Until now, this was done directly in the api config CREATE and DELETE
code, respectively, with a series of

if ($storage_type eq 'foo) {
    ...
} elsif ($storage_type eq 'bar') {
    ...
}

which isn't really that nice...

Another nice result of this approach is that also external plugins
can use those hooks and to their setup/cleanup steps sanely.

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