Rework `ScyllaLogFile.wait_for()` method to make it easier
to add required methods to ScyllaNode class of ccm-like shim.
Also, added `ScyllaLogFile.grep_for_errors()` method and
reworked `ScyllaLogFile.grep()`
We adjust all of the simple cases of cluster tests so they work
with `rf_rack_valid_keyspaces: true`. It boils down to assigning
nodes to multiple racks. For most of the changes, we do that by:
* Using `pytest.mark.prepare_3_racks_cluster` instead of
`pytest.mark.prepare_3_nodes_cluster`.
* Using an additional argument -- `auto_rack_dc` -- when calling
`ManagerClient::servers_add()`.
In some cases, we need to assign the racks manually, which may be
less obvious, but in every such situation, the tests didn't rely
on that assignment, so that doesn't affect them or what they verify.
