On severl instance types in AWS and Azure, we get the following failure
during scylla_io_setup process:
```
ERROR 2021-04-14 07:50:35,666 [shard 5] seastar - Could not setup Async
I/O: Resource temporarily unavailable. The most common cause is not
enough request capacity in /proc/sys/fs/aio-max-nr. Try increasing that
number or reducing the amount of logical CPUs available for your
application
```
We have scylla_prepare:configure_io_slots() running before the
scylla-server.service start, but the scylla_io_setup is taking place
before
1) Let's move configure_io_slots() to scylla_util.py since both
scylla_io_setup and scylla_prepare are import functions from it
2) cleanup scylla_prepare since we don't need the same function twice
3) Let's use configure_io_slots() during scylla_io_setup to avoid such
failure
Fixes: #8587Closes#8512
588a065304