Improve memtier-benchmark-02

[okartau]

In #816 a multi-memtier variant of memtier benchmarking was added.
This issue is to track and co-ordinate the follow-up improvements.
Two points were made in the #816, carrying those over here:

• Set the default CPU/mem requests and container counts so that the benchmark would run out of the box in the vm that is created from benchmarks.test-suite/memtier_benchmark/n4c16/topology.var.json, and possibly make the requests parameterizable with env variables.
• a way to force the benchmark to execute a fixed number of operations? That would be interesting when thinking of measuring total amount of energy needed for accomplishing the task. There is option --requests=NUMBER: total number of requests per client, should be evaluated can it be used for such a run.