Context cleanup

Before the fix of bug #1386932 (285cfaf0954d4c3e320b205c288240c1828476fe)
was committed, there are a few hacks in Cinder where an original copy
of (un-elevated) context has to be saved before doing
context.elevated().  Now that we have the fix in place, it's time to
clean up those old hacks.

Change-Id: Ie3e5cb7398647b4619d294c572e920e6c3b6b9c9
Related-bug: #1386392