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

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

Change-Id: I5e8df9e4bcd7a3c56887f6982e14dee2230ea641
Closes-Bug: #1529836

openstack/magnum-ui,openstack/magnum-ui,openstack/magnum-ui,openstack/magnum-ui