api: nodeconfig: validate acme config before writing

api: nodeconfig: validate acme config before writing

Currently it is possible to add the same domains as different
acmedomainX keys to the node config, which prevents the user from
ordering certificates later.

This patch adds a call to get_acme_conf, which does the semantic
validation (and is also used in all other sites, which read the
config).

Reported in our community forum:
https://forum.proxmox.com/threads/lets-encrypt-cert-on-gui-not-working.91014/

quickly tested in my setup, by successfully adding the same domain
twice without the patch, and failing to do so with it applied.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>