Add support for user-configurable parameters of query-types ratio

[yarongilor]

In order to test a more specific load scenarios like delete-intesive - the currently fixed query-type ratio can changed configurable for the user. it might be similar to the corresponding cassandra-stress user-profile mode.

For example, it'll be usefull to configure a high portion ratio for Gemni queries of type delete + read in order to test range-tombstones in a performance and high-stress fashion. It will be helpfull for testing a scylla improvement like: https://github.com/scylladb/scylla/commit/fce124bd9029a87dd70356eefe5ae16f406e06b3 .

Gemini current fixed query ratio is about 20(insert/update)/1-5(select)/1-5(delete). It may not be good enough for focusing the above case.