provision/downburst: ignore "username@" prefix in hostname

If downburst gets hostname as an argument which does not have
a username@ prefix (does not include @-symbol) there is an
index out of range error will occur, which we, obviously,
don't want and wish to allow hostnames don't use login names.

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

teuthology/provision/downburst.py

@@ -216,7 +216,7 @@ def build_config(self):
             'additional-disks-size': machine['volumes']['size'],
             'arch': 'x86_64',
         }
-        fqdn = self.name.split('@')[1]
+        fqdn = self.name.split('@')[-1]
         file_out = {
             'downburst': file_info,
             'local-hostname': fqdn,