Cloud-init support for LXC containers

[fthorns]

This patch allows user-data for cloud-init to be provided when defining an LXC container. Images containing cloud-init will use this data for initial provisioning. Once started, changes to cloud-init user-data usually doesn't make sense, therefore it is an option to just set once during the container creation. Feedback is welcome.

[matthewalanpenning]

Thanks for the pull request. I will do a little testing and plan to merge this in the next release.