Change references to org.apache.brooklyn for Maven groupId

Should be merged once the _org.apache.brooklyn_ artifacts are available on the OSS Sonatype Nexus server:

- https://oss.sonatype.org/#nexus-search;quick~org.apache.brooklyn
I don't think we'll be pushing `org.apache.brooklyn` to sonatype - I presume we'll push it to the apache nexus. @richardcloudsoft any thoughts on that, and what will the URL look of artifacts look like?

Also, if we wanted to support installing older versions then we'd need a conditional on the URL.
But I'm fine with it only supporting the more recent 0.7.0 and higher.