Explicitly specify the destination dir for images

I ran ramdisk-image-create from $TRIPLE_ROOT/tripleo-incubator and then
later setup-baremetal failed to find the deploy images in $TRIPLE_ROOT
where it expected to find them.

Seems safer to always specify the destination directory for image-create
and specify the full path to the images later.

Change-Id: Ibebd6544be9cc0b1224637b365891b9b9cf40589
Merge "Explicitly specify the destination dir for images"