Move utilities in the core plugin to the util module

This patch moves handle_api_error, MidonetApiExcption and
MidonetPluginException to midonet/neutron/common/util.py.

Change-Id: I011140d3f85f9c48e7d5cdbe5c5b85c7e400634f
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>