config: update bindings in property::reset and operator=

Previously we just set the value without notifying bindings. This
lead to ignoring property value updates when the property was reset to
its default value as a result of a patch_cluster_config request
with non-empty "remove" section.

(cherry picked from commit 7411586)

src/v/config/property.h

@@ -177,10 +177,14 @@ class property : public base_property {
         return validate(v);
     }
 
-    void reset() override { _value = default_value(); }
+    void reset() override {
+        auto v = default_value();
+        update_value(std::move(v));
+    }
 
     base_property& operator=(const base_property& pr) override {
-        _value = dynamic_cast<const property<T>&>(pr)._value;
+        auto v = dynamic_cast<const property<T>&>(pr)._value;
+        update_value(std::move(v));
         return *this;
     }