plugin id: limit to 'pve-configid' format

Else one can pass almost arbitrary data as ID and break editing or
deletion of a plugin.

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

src/PVE/ACME/Challenge.pm

@@ -13,6 +13,7 @@ my $defaultData = {
 	id => {
 	    description => "ACME Plugin ID name",
 	    type => 'string',
+	    format => 'pve-configid',
 	},
 	type => {
 	    description => "ACME challenge type.",