Reuse current DHCP address for admin interface

Change admin address allocator to re-use node.ip as address for admin
 interface.

This allows for substantially more usage of the admin network range sice it
 no longer needs to be divided in half to provde dedicated address range for
 DHCP and another range for admin addresses.

Duplicate addresses are un-likely to occur as DHCP assigned the address in
 the first place so we can assume that we can re-use this address for the
 life of the Node

Another change is needed in fuel-library and will be tracked shortly.

Closes-bug: #1274595

Change-Id: I1353f46b0cb0e6285fd03bd43ff26a4cba52a71e