Use correct hostname to get instances.

socket.getfqdn() would return a fully qualified domain name
which maybe invalid for nova to get instances by hostname.
If the host that runs devstack is virtual(like running
devstack on AWS), socket.getfqdn() may return a domain name
which is not the host name and nova cannot use that to get
instances running on current host.

Fix bug: #1166717

Change-Id: Ic6cc8607bcdeee720c5d46d44fbaa589b4fb950e
(cherry picked from commit 16f8e14b24006b23e1965329fa27ab7fcab3e6de)
Merge "Use correct hostname to get instances." into stable/grizzly