Reuse addresses from dhcp pool when possible

We can get rid from static pool, by reusing dhcp
address, assigned to node initially.

General procedure for assignment of ip addresses
is not changed, but i've added one more condition
to check if ip address is in range of admin_net.ip_range
if it is - it will be reused when creating ip address
if not - address will be used from pool, it will guarantee
that patch works on upgraded environments

Also made refactoring of old code

DocImpact
Closes-Bug: 1271571
Implments: blueprint get-gid-of-splitted-fueladmin-network

Change-Id: I1fcc4532faa6b255674b589ebd44534082e29c96
Merge "Reuse addresses from dhcp pool when possible"