Replace dev_string to dev_list

I've found out that when running the pvcreate command we were passing all devices as if they were one parameter like:

['/sbin/vgcreate', '-s', '4', 'vg0', '/dev/xvdb /dev/xvdc']

while the correct way would be:

['/sbin/vgcreate', '-s', '4', 'vg0', '/dev/xvdb', '/dev/xvdc']