Show maintenance field in GET /nodes

Before the maintenance field was only shown if client issued a GET on
a specific node (node-show), but this is an important attribute to have
listed in a GET /nodes (node-list), it's a good info to know which nodes
from that list are in maintenance mode or not, and the Nova Ironic Driver
also need this information to determine which nodes are available to be
exposed to Nova or not.

Change-Id: Icc31a312130a1c2d5853da64bd9091a93221363c
Merge "Show maintenance field in GET /nodes"