REVIEW NEXUS-6692: Remote URL Validation

Ancient code doing proxy repository URL validation is not sufficient. Especially as RRS powered by HttpClient 4.x uses java.net.URI too, and hence will choke on bad URLs not caught by core validation.

Issue
https://issues.sonatype.org/browse/NEXUS-6692

CI
http://bamboo.s/browse/NX-OSSF154