Write udev rules correctly when renaming multiple devices

bnc#894089h
Let us make sure that the methods we touch have comments, so that future reviewers can read only their comments instead of their implementation:
```ruby
# "A NIC with *dev_mac* will be named *dev_name*"
# @return [Array] an udev rule
def GetDefaultUdevRule(dev_name, dev_mac)
```