Remove a obsolete notice
write -> writing
Move code to show commands to ZabbixBenchmark class
Remove default command setting.

From now on users has to add a command line argument to run benchmark:

./zabbix_benchmark writing_benchmark

or

./zabbix_benchmark reading_benchmark
Don't register & remove dummy hosts on writing_benchmark.

From now on users has to run setup command before running benchmark:

./zabbix-benchmark setup