Added get log files from RDS
add the function get_all_log function
clean some codes
changed according to advice from  toastdriven
Adjust comments to match code in RDSConnection.get_all_logs()
Make optional RDSConnection.get_all_file argument optional
Added unit tests for RDSConnection.get_all_logs()
Merge remote-tracking branch 'pasc/rds_logs' into develop
Fixes #2040, #1994.