novajoin has usually just used the nova user for running and even file
permissions; however, as nova now supports passing the keystone token
for the vendordata plugin, and as novajoin should support being run in a
node (or container) where nova is not available, it makes sense to start
having an own user for this vendordata plugin service.
Thus, this commit adds that.
Change-Id: I190a84a5aaf1fcc301f0605931b24d5de6999a8b
bb6c415893