scylla_util.py: switch to subprocess.run()
scylla_util.py: switch to subprocess.run()

When we started to porting bash script to python script, we are not able to use
subprocess.run() since EPEL only provides python 3.4, but now we have
relocatable python, so we can switch to it.

scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla