Add info on autoloading kernel modules

This commit adds information in user.common.yaml about how to specify
a list of modules that should be added to /etc/modules and thus get
auto-loaded into the kernel at boot time.  It provides two recommended
defaults: the 8021q and vhost_net modules.

Implements: blueprint custom-modules-in-baremetal-provisioning
Merge pull request #39 from CiscoSystems/bp/custom-modules-in-baremetal-provisioning

Add info on autoloading kernel modules