computer/api node list should include per config.xml last-modified

Currently some information about all nodes is available at once via computer/api, but O followup requests are needed to get the full node config. In some cases it makes sense to include information from the config in computer/api (JENKINS-25286). But unless the whole config is piggy backed, there will always be something missing for someone.

By including a Last-Modified or hash of the config as a field in computer/api, it would be easy to for api clients to locally cache the node configs and avoid the O lookups