Fixes UNDERCLOUD_VM_NAME in instack-virt-setup
Fixes UNDERCLOUD_VM_NAME in instack-virt-setup

The instack-virt-setup uses UNDERCLOUD_VM_NAME to define the name
for the instack vm, but was hard coded to "instack" for finding
the IP address. This patch completed the usage of UNDERCLOUD_VM_NAME.

dpkshetty/instack-undercloud,eprasad/instack-undercloud,dmsimard/instack-undercloud,agroup/instack-undercloud,rdo-management/instack-undercloud,bcrochet/instack-undercloud,bcrochet/instack-undercloud,dmsimard/instack-undercloud,eprasad/instack-undercloud,agroup/instack-undercloud,rdo-management/instack-undercloud,dpkshetty/instack-undercloud