REST improvements

* `DelegatingSecurityProvider` reloads its delegate when the management context's properties are reloaded, so users can switch authentication modes or add/remove users without restarting Brooklyn.
* Adds the `brooklyn.rest.filter` package.
* Moves request logging from `BrooklynPropertiesSecurityFilter` to `LoggingFilter`.
* Quietens request logging significantly: `GET` and `HEAD` are logged at trace, `PUT`, `POST` and `DELETE` at debug. If an exception is thrown the request is logged at warn.