Fix typo'd format parameter in midonet_lib.py

Fix typo'd format parameter in midonet_lib.py

In add_static_nat(...):

        LOG.debug("MidoClient.add_static_nat called: "
                  "tenant_id=%(tenant_id)s, chain_name=%(chain_name)s, "
                  "from_ip=%(from_ip)s, to_ip=%(to_ip)s, "
                  "port_id=%(port_id)s, nat_type=%(nat_type)s",
                  {'tenant_id': tenant_id, 'chain_name': chain_name,
                   'from_ip': from_ip, 'to_ip': to_ip,
                   'portid': port_id, 'nat_type': nat_type})

Note port_id vs portid.  This line of code raises a KeyError if debug
logging is enabled.

(Found via pylint)

Change-Id: I4d76255a559489f2f0e074b4489cfa5f33f1dddf
Closes-Bug: #1404755