Allow old alt*DeploymentRepository property format if default layout is used

In 3.0.0-M1, the format of the alt*DeploymentRepository properties changed from id::layout::url to id::url

It would be awesome if the old format was supported if and only if the layout was equal to default.  Specifically, the following formats should be supported:

• id::url
• id::default::url

This would allow for better backwards compatibility, and to allow the same property values to be used regardless of what version of the maven-deploy-plugin was used by a project (as long as the layout is default)

In my case, we have a global maven settings.xml file on our CI server that specifies the alt*DeploymentRepository properties to deploy to our corporate maven repository.  I would like to be able to use this global maven settings.xml file regardless of what version of maven-deploy-plugin is used by our (many) projects.  In order to do that, I would like to use the id::default::url format.