add support proxying maven.oracle.com in nexus 3

Acceptance

• Nexus 3 maven2 proxy repo configuration should have an option for supporting circular redirects (as NX2 does)
• This setting should be migrated from NX2 during maven2 proxy repo migration.
• This feature should be documented in the book.

I configured my maven-oracle repo after migration to have the right credentials but I am not sure how to configure the rest of the setup so it continues to work as it did in 2.x.

See

http://maven.oracle.com
and https://support.sonatype.com/hc/en-us/articles/213465728-How-to-configure-a-proxy-repository-to-maven-oracle-com
for more info.

At the very least it should throw some sort of warning during the migration if this circula redirect is configured for a repo.