Don't unnecessarily loop through all ports/interfaces

Don't unnecessarily loop through all ports/interfaces

The ovs-vsctl 'list' command can take a list of records as an
argument, so there is no need to manually loop through all records
discarding the ones with names that don't match the bridge's
port name list.

Also, since the data is returned as json, the ofport returned isn't
a string, so we don't have to convert it to int for testing.

Closes-Bug: #1404935
Change-Id: Ica20f8f8315196aa5a04657d28dde41a75f0368d