XD-2148 XD-2149 add distShellZip gradle task

- remove unused libs from shell distro

- update xd-shell and xd-shell.bat

- add jars from spring-xd/xd/lib
Fix RabbitMessageBusTests

The AMQP endpoints now hold expressions internally as a
SpEL Expression instead of a String. Some tests were
examining these Strings.

Change tests to use expression.getExpressionString().
Fix enableMessageRates property type

 - Use `String` instead of `boolean`