Avoid AttributeError in servers.Server.__repr__

servers.Server represents various object now, and some of them may
don't have attribute 'name', for example, the interface_list() result
object. It will cause AttributeError when we try to format string with
such object, so I add a check for the 'name' attribute in __repr__
method, it will use 'unknown-name' instead when 'name' is not found.

Change-Id: If4757d5d73721774543d58a4cc875710a6013f34
Closes-Bug: #1280453
Merge "Avoid AttributeError in servers.Server.__repr__"