Support for customizing list of kernel modules loaded at boot

We currently provide a couple of kernel-related items as part of our preseeding process: you can specify a particular kernel package that you want to be the default at boot time, and you can provide custom command line options to the kernel. Both come via user data that gets translated into actions occurring in the late_command section of the preseed during baremetal provisioning.

We have a request for another kernel-management feature: the ability to add a custom list of kernel modules that get loaded at boot time. This would allow, for example, for the vhost_net module to be loaded at boot time for OpenStack to take advantage of more performant networking.

We actually already have a bit of code that ensures the 8021q module gets loaded at boot time, so all that would required here would be to turn that from a static addition to a parameterized list and expose it via user data.