Always attempt to clean port device_id's on reboot

Recently I have come across a few cases where device_id is still set to
an id that does not exist anymore. Somehow the vm was shut down previously
and the port device_id did not get cleaned properly. This change will force the
rug clear device_id on all router ports before a reboot is attempted.
Only clear device_id on reboot if one is already set
Merge pull request #194 from jordant/master

Always attempt to clean port device_id's on reboot