Make non-admin users can unshelve a server

If non-admin users try to unshelve a server, nova-compute fails due to
"User does not have admin privileges" while getting hypervisor_hostname.
This patch changes the user context to admin's one temporarily to get the
hypervisor_hostname.

Closes-Bug: 1245312
Change-Id: I84dce1f7694d09c1281da13c6008bb3aa5940f85
Merge "Make non-admin users can unshelve a server"