ovs: Make interface name hashing algorithm common and extend it

ovs: Make interface name hashing algorithm common and extend it

The OVS device name hashing algorithm shortens interface names that are too
long. To ensure uniqueness it makes use of a hashing algorithm.

Move this function to a common place where it can be shared between ml2
drivers and agents.

Extend the function to support defining the max device length to be used.

Change LOG level to info to help deployers figuring out the unhashed name
of an hashed inteface.

Adapt OVS agent to use this common function instead of its
own implementation.

Change-Id: I5c04f39928d070aa7e372934fcb2675609d2761c
Partial-Bug: #1495960