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: I2ff32c4dc37c543696125ac755dc4adb69ddacdf
Partial-Bug: #1529836

klmitch/neutron,igor-toga/local-snat,mahak/neutron,MaximNevrov/neutron,noironetworks/neutron,huntxu/neutron,wolverineav/neutron,openstack/neutron,mahak/neutron,igor-toga/local-snat,bigswitch/neutron,cloudbase/neutron,openstack/neutron,eayunstack/neutron,mahak/neutron,bigswitch/neutron,dims/neutron,sebrandon1/neutron,dims/neutron,klmitch/neutron,MaximNevrov/neutron,huntxu/neutron,eayunstack/neutron,openstack/neutron,cloudbase/neutron,sebrandon1/neutron,noironetworks/neutron,wolverineav/neutron