Refactor rebind code, and tidy

Please merge https://github.com/apache/incubator-brooklyn/pull/308 first
Not sure I follow. `newManagementContext ` gets terminated in `tearDown()`.
Until my changes, the `newManagementContext` would only get set in some of the methods but not others (which definitely felt like a bug). 
Now everything delegates to a single rebind() implementation, which does set `new ManagementContext` - so it should always happen.
Setting it in the sub-class, or declaring your own locally-scoped variable/field, will still work but should be unnecessary.