Change in pre_first_boot behaviour

[Graham Gilbert]

Firstly, apologies for posting this to both imagr-discuss and imagr-dev, but I thought it was an important enough change in behaviour that I wanted to make sure everyone saw it. Previously, if you wanted to run scripts or install packages during the Imagr run, you set pre_first_boot to true. This wasn't very well thought out by me, so I'm changing it before we get too far along.

Following the latest commit, you now should set first_boot to false if you want your scripts or packages to be used during the Imagr run. first_boot is assumed to be true if it's not set.

Thanks,

Graham