Replace deprecated library function os.popen() with subprocess
Replace deprecated library function os.popen() with subprocess

os.popen() is deprecated since version 2.6. Resolved with use of
subprocess module.
Closes-bug: #1529836

Change-Id: Ibdb8f13462e8416097006e19eb2214425364910e

openstack/octavia,openstack/octavia,openstack/octavia