XD-1630: Cut dep from xd-shell to xd-dirt

XD shell should know about dirt only through the REST API.
lessen the dependency to test. As a side effect, changed handling of container attributes as a plain map.

@ilayaperumalg may want to have a look at the Xerces stuff which I left commented out, as it may have been a problem with the fact that shell depended on dirt in the first place.

Also, the dependency to spring-xd-test does not seem to be necessary!?

Lastly, but this may be the subject of another Jira, we may want to use jackson2 in the shell thoughout