During install when finding an empty <rolls/> tag, use all pallets from the DVD

[bsanders]

For automated frontend installs from a DVD, it would be useful to be able to specify a rolls.xml file that tells the installer to install everything on the DVD, without the front-est-end knowing the contents of that DVD.

It seems that if /tmp/rolls.xml is essentially:

<rolls></rolls>

It could be used to interpret this. If any other stacki devs know this would break something else (or have a better suggestion), please let me know.