Expose created_at/updated_at properties in the REST API

Ironic client display created_at/updated_at values, but API does not
return these fields, so node-show display empty value always. With
this fix, we will add created_at/updated_at two fields into API base
class to enable all API to support these two fields, including node,
port and chassis now.

Closes-Bug: #1244072
Change-Id: I03f935902a86fac2f8fb58703c72ca0fda84843b
Merge "Expose created_at/updated_at properties in the REST API"