Use .png instead of .gif images

This replaces the two image files used in the Admin Guide, "Introduction
to Ironic" page. The images were in .gif and are being replaced with
their .png counterparts, because .png is a better format.

Change-Id: Iec831d37f58f4e159b0cfc197bac5dad433d32cf
Merge "Use .png instead of .gif images"