Migrate cloud image URL/Release options to DIB_.

This switches $CLOUD_IMAGES and $RELEASE to the DIB_ namespace so
they will survive future changes to the sanitisation of the build
environment.

Change-Id: I7dc2aa82fb9ef452705b080cc404f41046014f20
Grab the next available /dev/nbdX.

Change-Id: I6a2c8e58cdab889d747e870d0009f51bba0ff148
Merge "Grab the next available /dev/nbdX."
Merge "Migrate cloud image URL/Release options to DIB_."