Automatic image download for local images.

This adds the parameter 'url' to rz_image. If the source path doesn't exist
and url is set, then the image will be downloaded and saved to the path. This
eases the automatic setup of Razor installations and reduces the network
traffic load caused by large setups.

By providing the url as an additional parameter it is possible for users to
control where local images are stored and still have them automatically
downloaded. It is important for users to have control over this path as it may
be highly specialized for their environment.
Make download fail on HTTP error.

Enable curl error reporting on HTTP errors. Without this flag curl can
silently fail to download an image and will result in an html file (instead of
the expected ISO). Users currently get no notification of this and instead
find that their systems won't properly netboot with VirtualBox indicating that
the VM has attempted an invalid op.
Use the latest mk by default.

This changes the default mk image to use 'latest' instead of a specific
version. Using 'latest' will reduce the maintenance burden with this setting.
It will only need to be updated in the future if the base location of the ISO
changes.