NEXUS-2450: [M1 to M2] EJB artifacts are served from **/jars/ directory instead of **/ejbs/

Fixes
https://issues.sonatype.org/browse/NEXUS-2450

Use case: using Maven3, and you need to access some legacy Maven1 layouted repository with EJBs. As Nexus has not enough information (especially the "type" of the artifact in question) only the path, this change simply makes Nexus to iterate one extra M1 folder when looking for artifact. This happens in doRetrieveItem method.

No other change, despite introduction on List<String> as return params on coversion methods, as all the effected metods (naturally except the doRetrieveItem) just use the 1st element of the list, retaining same behaviour as before (when only single string was returned).