Ignore non-existent ports during OVS intf list

Ignore non-existent ports during OVS intf list

A recent commit[1] to pass the list of port names directly to
ovs-vsctl during a list operation introduced a new possible
failure condition where one of the names might refer to a port
which no longer exists. By default this causes ovs-vsctl to quit
in a fit of rage[2].

Previously, all interfaces were retrieved and the ones that were a
subset of the name list were processed. The name list could contain
extra non-existent names (e.g. recently deleted interfaces).

This patch just passes the '--if-exists' flag to the 'list Interface'
command to match the same previous behavior.

1. 3f0bf6cfac2e151d5a4a7f076062b3365bdbf457
2. Example: Stderr: 'ovs-vsctl: no row "tap80664420-ea" in table Interface\n'

Closes-Bug: #1407190
Change-Id: I8f359981386d13fb455281a72b8bb245395c0909