Settings: make global config file optional

src/settings/global_user.rs

@@ -22,9 +22,15 @@ fn get_global_config() -> Result<GlobalUser, failure::Error> {
     let config_str = config_path
         .to_str()
         .expect("global config path should be a string");
-    s.merge(File::with_name(config_str))?;
 
-    // Eg.. `CF_ACCOUNT_AUTH_KEY=farts` would set the `account_auth_key` key
+    // Skip reading global config if non existent
+    // because envs might be provided
+    if config_path.exists() {
+        s.merge(File::with_name(config_str))?;
+    }
+
+    // Eg.. `CF_API_KEY=farts` would set the `account_auth_key` key
+    // envs are: CF_API_KEY and CF_EMAIL
     s.merge(Environment::with_prefix("CF"))?;
 
     let global_user: Result<GlobalUser, config::ConfigError> = s.try_into();