Avoid race condition when setting default gateway.
If ip link set is failed, then try brctl addbr.
Set BRTYPE to "linux" by default.

If the bridge doesn't exist, `BRTYPE` wasn't set, and the code to
add the interface to the bridge was never called.

Set it to `linux` by default.
Shorten interface names to avoid too long names

This "fixes" #28
Give our ARP neighbors a nudge about the new interface

The new interface cannot be reached until the local switch learns about
its MAC address, which could take minutes. Sending a gratuitous ARP
moves things along so that the interface becomes reachable right away.
Fix typo in warning message
Update deprecated docker flags to new format