list_instances() to return a list of instances names

Previously the list_instances() method in the Nova Ironic driver was
returning a list of instances UUIDs instead of a list of instances names
causing various checks throughout nova.compute.manager to always return
False for Ironic.

Closes-Bug: #1309719
Change-Id: I0ed29911f180f99a7e2b1c00ddd6c43064f04f7f
Merge "list_instances() to return a list of instances names"