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
55e3df8a72