Ensure a newline exists for each interface config

For some reason it seems that a newline is required for some of the
interfaces to come up when executed by /etc/netstart.  This code ensures
that the implementation of bsd::network::interface for openbsd will
append a newline to the config files.
Merge pull request #18 from puppetlabs-operations/newline

Ensure a newline exists for each interface config