Change the half of the bridge name used for ports

Change the half of the bridge name used for ports

The code to generate the names of the patch ports
was based on a chunk of the bridge name starting from
the beginning. With the long suffix, this ended up
excluding all of the random characters in the name.
(e.g. br-int374623235 would create an interface br-in-patch-tun).

This meant that if two tests using patch interfaces ran together,
they would have a name collision and one would fail.

This patch updates the patch port name generation to use the
randomized back portion of the name.

Change-Id: I172e0b2c0b53e8c7151bd92f0915773ea62c0c6a
Closes-Bug: #1470637