[2.8.x] NEXUS-6356: RUT Realm might mis-idenfity user

As searchUsers were initially implemented for UI user search
use case, where "prefix search" is done.

Backport of 
https://github.com/sonatype/nexus-oss/pull/426

Issue
https://issues.sonatype.org/browse/NEXUS-6356

CI
TBD