OVS Agent: limit veth names to 15 chars

The maximum length of a veth interface name is 15 characters.
This patch limits the length of the veth names created for
integration bridges by switching to a fixed-length hashing
mechanism if the normal name would exceed the allowed length.

Closes-Bug: #1328288
Change-Id: I432cee62a6dc91f268becbc91f8024c23dd0bfac
(cherry-picked from 1222366f6dadf7ce2a810c919344054196134db8)
Merge "OVS Agent: limit veth names to 15 chars" into stable/icehouse