Added support for setting custom MAC address to guests

This patch allows the users to specify the MAC address
of the guests instead of using the default generator
included in Oz. This can bring some flexibility while
customizing the guests OS in networks with non-trivial
DHCP configurations.
tag cached JEOS images with a format-specific extension

Without this, changing the image_type with cached JEOS images present will cause
breakage.  We will pull in a cached JEOS even if it is of the wrong type.  This
does not end well.
Be explicit about which JEOS file we are using
Add in checks for Ubuntu desktop CDs.

Unfortunately, Ubuntu desktop CDs do not contain
the openssh-server.  Therefore, we cannot do
customization or ICICLE generation if the desktop
CD is in use (we can still do a basic install).
Instead of failing *after* the installation,
instead do a check right after unpacking the ISO
and throw an exception there.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>