Clean up OvsdbPortUpdateCommand.java

This change abstracts the different steps involved in the
OvsdbPortUpdateCommand.execute() method so that the code is more readable. The
individual steps have been put into the appropriate private helper methods.

Change-Id: Ifacdc0fe44f0555da690ca1c306b0462d34bc24e
Signed-off-by: Ryan Goulding <ryandgoulding@gmail.com>