scylla_cloud_io_setup: move aws io preset values to external YAML file

Since we support many instance types on AWS EC2, handling all instance
types by "if - elif" conditions is difficult to read and hard to maintain.

We should move out these preset values to the external file.

Note that, in the original parameter definition, " * nr_disks" was applied to
some instance types, but the new code applies it to all instance types.

This is because we know that instance types to which does not applied
" * nr_disks" have only one disk.

Fixes scylladb#544