Clarify code in nodepool/cmd/builder.py
Clarify code in nodepool/cmd/builder.py

Clarify code in nodepool/cmd/builder.py so we don't have two classes
named NodePoolBuilder and multiple variables named 'nb'.

Change-Id: Icb77ca2edea49ed3b787e600cf3754160ee1eff8

Tesora/tesora-nodepool,Tesora/tesora-nodepool