Add downloadonly flag to fedora

devstack-gate needs to be able to make an image that has pre-downloaded
but not installed a sequence of packages. To support that, add a -d flag
to install-packages to allow downloading but not installing. -d passes
through on a dpkg install, and with this patch should be transformed
to --downloadonly which is the yum version.

Change-Id: Ia4e61fa4304df86afbee7dfc8067ea1bdf528f3e