Comment 50 for bug 1738935

Reviewed: https://review.opencontrail.org/38737
Committed: http://github.com/Juniper/contrail-test-ci/commit/ea67dfefb22611b2d3c18e1fad32f82efc82c74b
Submitter: Zuul (<email address hidden>)
Branch: R4.0

commit ea67dfefb22611b2d3c18e1fad32f82efc82c74b
Author: ankitja <email address hidden>
Date: Fri Dec 29 15:40:40 2017 +0530

Add a check to make sure pcap file is getting created

Closes-Bug: #1738935

Intf Mirroring cases failing in the sanity, but passed on rerun.

Possible cause for the intermittent failure in the sanity could be
the pcap file is getting deleted after starting tcpdump.

In the start_tcpdump method, we first delete already existing file before creating a new one.

To make sure that the pcap file is present after starting tcpdump we need to
make sure that the pcap file is actually deleted if already exists before starting tcpdump.

This commit might solve the intermittent issue in the sanity.

Change-Id: I2297433f25dbe1f8bae8c01e4601baaaee1290b9