Allow setting port id

Allows the user to specify the port id when creating a port through
the midonet API.

Converter.java: copys over the id of the port to the config
PortZkManager.java: only generate a new UUID for the port id if it is
    not already set.

Change-Id: I229df492b4b9368214abd97ba0d9e41882f48d9c
Signed-off-by: Joe Mills <joe@midokura.com>
Merge "Allow setting port id"