misc: allow _ssh_keyscan return None again

A host may not have any keys, so _ssh_keyscan should return None,
instead of failing with error:

    IndexError: list index out of range

Fixes: dbd55e3

Signed-off-by: Kyr Shatskyy <[email protected]>

teuthology/misc.py

@@ -1127,7 +1127,8 @@ def _ssh_keyscan(hostname):
     for line in p.stdout:
         host, key = line.strip().decode().split(' ', 1)
         keys.append(key)
-    return sorted(keys)[0]
+    if len(keys) > 0:
+        return sorted(keys)[0]
 
 
 def ssh_keyscan_wait(hostname):