Fix nova-compute start issue after evacuate

After evacuated successfully, and restarting the failed
host to get it back, Nova will call init_host() and then
call method _destroy_evacuated_instances(). In method
_destroy_evacuated_instances(), nova will check again if
the storage is shared or not to decide if the storage
should be destroyed. Now nova is using temp file to check
if it's shared file system, but it's wrong for RBD case.
So Nova will attempt to delete the shared block storage,
which will fail since it's used by the new instance. This
patch fixes this issue and adds test cases for that.

Closes-Bug: 1385484

Change-Id: I71bb818f3c2930b3a2ddf1817dfd4bb61fae7e98
Merge "Fix nova-compute start issue after evacuate"