Image builder update

* Installation script makes full system preparation
* README updated
* Functions moved to a separate file
* A few helper scripts added
* Ability to modify autounattended files added to Makefile
* Config file updated
* Unattended files updated
* Obsolete files removed
* VirtIO URL and file name updated
* A section for Windows Server 2012 R2 added
* Minor updates in functions

This change also includes modifications from https://review.openstack.org/#/c/68894/

Related-Bug: 1230471

Change-Id: If359789737221c1ab6a19d060d1c45dcf9866282
Merge "Image builder update" into release-0.4