[NEXUS-5806] Always set the xml:base

https://issues.sonatype.org/browse/NEXUS-5806
https://bamboo.zion.sonatype.com/browse/NX-YUMF4

By setting the xml:base the mergerepo will be able to correctly set the xml:base on the merged metadata. Without the xml:base will be pointing to file:/// urls on the local filesystem of nexus server.