We somewhat discussed skipping qcow2 generation previously in
I9372e195913798a851c96e62eee89029e067baa1. As recent issues with PPC
testing have shown, we are not actually testing the "vm" element and
hence the bootloader path in the functional tests.
I don't think we need to test this on every element; it overlaps
somewhat with the testing done by the nodepool jobs which build full
images and boot them. I also didn't want to introduce a separate run
for this. Thus it seems valuable to at least have one element
enhanced to do this installation and conversion in our default tests
for basic sanity.
This disables qcow generation by default, as per the other change, but
allows an element to drop a file that will override the output
formats. The Xenial element is modified to produce a qcow2 using
this, and also introduces a dependency on the "vm" element so it tries
to install the bootloader.
We now exit if the .qcow2 fails to build as well.
Change-Id: I1a6acefe52f8c696c39b2d592fdc7ae32a87e6fe
