Big Switch: Check for 'id' in port before lookup

Check for the presence of the 'id' key before trying
to do a lookup based on the value. This is necessary
because a 'fields' param to get_port(s) may exclude
the 'id' field before the host_id lookup.

Closes-Bug: #1358668
Change-Id: If30567d5deaeabd2de6c6287fac7a81695a41cc5
Merge "Big Switch: Check for 'id' in port before lookup"