Improve IT stability.

Instead of start-stop-replace-start, just start
the needed server once by overriding the method in
superclass, instead to rely on JUnit annos.
NEXUS-5861: RemoteStorageContext values might come from parent

So, wrapping the one of the 3 values as read only.
As other two (proxy and auth) are only at global level
(no more per-repo proxy, only global, and auth was never
global, only per-repo).
Merge pull request #114 from sonatype/nexus-5861-rsc-parent

NEXUS-5861: RemoteStorageContext values might come from parent
Merge remote-tracking branch 'origin/master' into NEXUS-5490-remote-user