A space-separated list of paths to RPMs. You can supply either URLs
from which we should fetch the RPMs, or paths to files on
users.emulab.net. For example:
http://www.myU.edu/~me/myrpm.rpm
or
/proj/myproj/rpms/myrpm.rpm
These RPMs will be installed in all slivers in the slice, and you can push
out updates to them while the slice is running.