fix(upgrade_test.py): backup/recorer mount config
fix(upgrade_test.py): backup/recorer mount config

This patch added two helper functions to backup and recover scylla
config files.

Two mount config files won't be removed in uninstallation from 4.6/2021.1,
so let's backup them for rollback.

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests