[JENKINS-14999] Check permissions with authorization of the triggering user.

This is a improvement for [JENKINS-14999] (https://issues.jenkins-ci.org/browse/JENKINS-14999).

When specify a project to copy artifacts from using build parameters, permission check is performed at building time.
Copy Artifact allows to copy artifacts only if the project can be read from any authenticated users.
It prevents to copy artifacts from projects allowing to be accessed from particular users.

This patch makes Copy Artifacts to check permissions with authorization of the triggering user.

I want my changes reviewed as I don't know much about authorization mechanisms of Jenkins.