Fixing the deprecated library function.
Fixing the deprecated library function.

os.popen() is deprecated since version 2.6. Resolved with use of
subprocess module.

Change-Id: I0751dbda085922d5e390cb7a19a9e832d399e20b
Partial-Bug: #1529836

bigswitch/tempest,Tesora/tesora-tempest,openstack/tempest,Juniper/tempest,masayukig/tempest,vedujoshi/tempest,LIS/lis-tempest,sebrandon1/tempest,cisco-openstack/tempest,LIS/lis-tempest,bigswitch/tempest,Juniper/tempest,sebrandon1/tempest,Tesora/tesora-tempest,zsoltdudas/lis-tempest,cisco-openstack/tempest,vedujoshi/tempest,zsoltdudas/lis-tempest,masayukig/tempest,openstack/tempest