Add the ability to run custom scripts inside ubiquity-dm

[rational]
Not a bugfix but something that seems to be important for OEMs. As 12.04.1 is the current LTS release, it seems worth including. The code itself is pretty clean and self contained.

[test case]
1) sudo stop lightdm
2) sudo stop ubiquity
3) Dump a script in /usr/lib/ubiquity/dm-scripts/{install|oem}
4) sudo start lightdm

5) Check that the script is indeed executed

[regression potential]
The feature isn't used in Ubuntu itself so I'm not expecting any breakage there, it could be that something in the added code goes wrong but that'd be specific to the OEM usecase where one of these scripts is present.

In our work on certain OEM projects we found that it has been beneficial to run simple, custom scripts inside of ubiquity-dm.

For example, we have a python script that we have currently been running during OOBE / OEM-config as a ubiquity plugin that plays an animation during firstboot before the language page loads. This has worked OK, but, there is a long gap between the ubuntu splash screen and when the animation fires up.

In order to cut down on this gap, we found that if we run the script inside of ubiquity-dm it really tightens up the amount of time that it takes for the video to start playing. In the meantime, the rest of OOBE loads up in the background and the user is immediately presented with the language page as soon as the animation finishes up. In many cases, there is little to no gap anymore between splash screen and animation kickoff.

This led us to propose a patch to ubiquity that will allow a user to place a simple, custom script of their choosing into either a /usr/lib/ubiquity/dm-scripts/install or /usr/lib/ubiquity/dm-scripts/oem directory. Depending on whether the user wants to run the scripts in either ubiuquity-proper or oem-config, some logic was added to the ubiquity-dm script to walk the above directory paths and launch any scripts located in the respective directory.

I will be attaching my launchpad branch that contains the requested changes for uqbiquity. Any improvements, suggestions are welcome. I would ultimately also like to get this into precise -proposed.

Thanks.