dist: scylla_util: prevent IndexError when no ephemeral_disks were found
dist: scylla_util: prevent IndexError when no ephemeral_disks were found

Currently we call firstNvmeSize before checking that we have enough
(at least 1) ephemeral disks.  When none are found, we hit the following
error (see #7971):
```
File "/opt/scylladb/scripts/libexec/scylla_io_setup", line 239, in
if idata.is_recommended_instance():
File "/opt/scylladb/scripts/scylla_util.py", line 311, in is_recommended_instance
diskSize = self.firstNvmeSize
File "/opt/scylladb/scripts/scylla_util.py", line 291, in firstNvmeSize
firstDisk = ephemeral_disks[0]
IndexError: list index out of range
```

This change reverses the order and first checks that we found
enough disks before getting the fist disk size.

Fixes #7971

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

Closes #8027

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