changes to support imgfac plugin

Change-Id: I5c39fcba6b81550bcf5e0f2d193c4aa234376907
move exit call to nova-install from Builder

Change-Id: Id3e504bbaeb1c40a940185e252e02166324fc17c
download the image into a temp file and hand it off to caller

Change-Id: I3fa380eea1a075abd17d26d6179e798a16f81411
Merge pull request #12 from redhat-imaging/imagefactory-324

Imagefactory 324