Git external repo fixes...

- re-enabled polling logic in case of git external repo
- unified configuration constants.
- fixes #2514 

@iocanel and @tdiesler  can you give it a quick look?
**ITests are ok** and I have manually tested with and without an **external Github repo**. It **worked fine** in my case.

Th reason why I have renamed the `configuredUrl` in https://github.com/paoloantinori/fabric8/blob/48e5085d16e44f478391dfad029b22287fd3bbd6/fabric/fabric-git/src/main/java/io/fabric8/git/internal/GitDataStoreImpl.java was just to unify with the `pid` property and because `@Property` annotation didn't allow me to have different keys for the variable name and the `name` attribute.