ansible: Update path for downloaded ISO images

ansible: Update path for downloaded ISO images

/var/lib/libvirt/images isn't world-readable by default. The ISOs are
meant to be readily accessible as they should be used read-only.

In the cases where VMs are run as unprivileged users (eg. with lxd),
/var/lib/libvirt/images/*.iso isn't readable. Rather than loosening
the mode on the images folder, the ISOs will be in a directory
that is world-readable by default.

Change-Id: I056c290e5b88f0baaa52eaf749d4f29b53dbb8af