Add repository url on Archetype / ArchetypeVersion

We need to be able to define a non-default repository hosting the archetypes.

Use case : a new archetype (not-sync with central) is used, we need to know where to get it from (jboss nexus repo).

An optional repositoryUrl could be set on the Archetype model AND on ArchetypeVersion :
In order to d/l an archetype, we check if it has a repositoryUrl, if it doesn't we check if its parent archetype has one. If none is defined, we assume it comes from Maven Central.