Allow to pass config files to cloud-init device

[lstocchi]

#123 adds a first implementation to pass configuration to cloud-init so that it can set the system at boot time. However it just allows to pass an ISO image which must be built by the user in advance.

It could be useful to give users the chance to pass just their config files and make vfkit create the iso image itself.

So far we have
--device cloud-init,path=<iso_image_path>

What we could offer is
--device cloud-init,user_data=<file_path>,meta-data=<file_path>....