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