Updating the vanilla image building docs

Changes
* moved some text around, namely the intro paragraph
* adding a link to devstack docs and referencing the config file
* adding a CentOS to the list of default actions that
diskimage-create.sh will perform
* adding a link to the official diskimage-create doc page
* general cleanup

Partial-Implements: blueprint update-docs-icehouse

Change-Id: Iddb37db9d03c58d0573f2ae872d0f89f388ac41e
Merge "Updating the vanilla image building docs"