fix(controller): standardize API datetime field format

This brings the datetime field's formatting more in line with the rest
of the platform.

From http://www.django-rest-framework.org/api-guide/fields#datetimefield:

> format - A string representing the output format. If not specified, this defaults to None, which indicates that Python datetime objects should be returned by to_native. In this case the datetime encoding will be determined by the renderer.

In our case, the datetime fields were being output in ISO-8601's format, which is different compared to the rest of the platform's datetime format. deis/logspout shares this format.