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
(cherry picked from commit 13fa2caaad796ee66c432e0d19fd6f759234a964)
Merge "Make non-admin users can unshelve a server" into stable/havana