[NEXUS-5806] Always set the xml:base

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
Merge remote-tracking branch 'origin/master' into NEXUS-5806-always-use-base-url

Conflicts:
	nexus-yum-plugin/src/main/java/org/sonatype/nexus/yum/internal/task/GenerateMetadataTask.java
Merge pull request #25 from sonatype/NEXUS-5806-always-use-base-url

REVIEW: [NEXUS-5806] Always set the xml:base