wrap generate_install_media in a try/finally block
wrap generate_install_media in a try/finally block

Without this any failures in generate_install_media will result in the builder
never releasing the lock.

jmcabandara/imagefactory,LalatenduMohanty/imagefactory,jmcabandara/imagefactory,LalatenduMohanty/imagefactory,henrysher/imagefactory,redhat-imaging/imagefactory,redhat-imaging/imagefactory,henrysher/imagefactory