Revert "Update plugins/dstat_mysql5_cmds.py"

This reverts commit fe5f91621f9c37671f359a75b40f4d6c29dd82e6.

I cannot understand why this file must take only two environment variables.
Other files about MySQL5 can take many environment variables.
Therefore, I guess that fe5f91621f9c37671f359a75b40f4d6c29dd82e6 is a mistake.