scylla_setup: warn users on the severity of answering no to IOTUne
scylla_setup: warn users on the severity of answering no to IOTUne

The system won't work properly if IOTune is not run. While it is fair
to skip this step because it takes long-- indeed, it is common to provision
io.conf manually to be able to skip this step, first time users don't know
this and can have the impression that this is just a totally optional step.

Except the node won't boot up without it.

As a user nicely put recently in our mailing list:

"...in this case, it would be even simpler to forbid answering "no"
 to this not-so-optional step :)"

We should not forbid saying no to IOTune, but we should warn the user
about the consequences of doing so.

Fixes #4120

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>
Message-Id: <20190121144506.17121-1-7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla