Fix libvirt evacuate instance on shared storage fails

When evacuate instance which has config drive enabled on shared storage,
disk.config file can't be created because of permision error, chowns
disk.config if it already exists.

Change-Id: I20bafc3482ead30297ad5402a09e97141816f4d1
Closes-Bug: 1247526
(cherry picked from commit 3dfa2f9fd4a3084ae3fb8a7ba5d8a7458ae1c3bc)
Merge "Fix libvirt evacuate instance on shared storage fails" into stable/havana