Cycles preview scene: made render layer name match name used for BI preview

This is needed because render result is being reused between different render
previews and keeping names out of sync means once preview result was created
for blender internal, it wouldn't be updated after switching to cycles.

Same goes to switching from cycles to other renderers like lux and yaf.

Simply updated cycles_preview.blend for now, could be solved from code side
as well, but IMO better to collect such a limitations of preview blend files
and solve them all together during "custom preview.blend for external engines"
project.

Reported by Jens in IRC.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54290 954f8c5b-7b00-dc11-b283-0030488c597c