Add the missing psutil dependency

[skorotkov]

Now the psutil module used not only in the unit tests.

So move it to requirements.txt

Fixes the https://github.com/confluentinc/ducktape/issues/419

[confluent-cla-assistant[bot]]

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[sprsi]

@skorotkov Was it throwing some error without this change?

[skorotkov]

Yes, see the https://github.com/confluentinc/ducktape/issues/419

[ivandasch]

@tsrivatsavs Could you please take a look? Also, the patch looks good to me