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
Again, this is the wrong URL for `org.apache.brooklyn` snapshot artifacts (I'm guessing).

There is also a chicken-and-egg problem. When we next bump the version number to 0.8.0-SNAPSHOT, then this artifact will not be in apache nexus (or sonatype) so the test will fail. The version would (by default) not be pushed until all tests pass. So it would require manual intervention to fix. Should we just live with that? If so, we should note it in the release process docs.