Allow not to have hooks or groups

Before this commit, the configuration would be considered invalid if at
least one hook and one group were not present. For obvious reasons,
groups, which are simply an ordered aggregation of hooks, should be
optional. On the other hand, hooks may seem mandatory since it is the
only way to validate a challenge. However, this is false in some
circumstances, for example in a test environment when running the client
against a test server that does not perform challenge validation.

https://github.com/letsencrypt/pebble#skipping-validation

acmed/src/config.rs

@@ -12,7 +12,9 @@ use std::path::Path;
 pub struct Config {
     pub global: Option<GlobalOptions>,
     pub endpoint: Vec<Endpoint>,
+    #[serde(default)]
     pub hook: Vec<Hook>,
+    #[serde(default)]
     pub group: Vec<Group>,
     pub account: Vec<Account>,
     pub certificate: Vec<Certificate>,