Limit interface names to 15 characters

If you give your bridge a long enough name, eg. `br-external`. The
extra characters added to the veth pairs can make the interface name
go beyond the 15 character limit. We can solve this by truncating the
name of the bridge used in the veth names.

Change-Id: I5b890e24195d033897a597a0a93a1cacfb2030d2