run: do not import at top level

This makes a run with --lock hang when connecting, for some
reason.  E.g.,

 $ teuthology -v a.yaml --lock
 ...
 INFO:teuthology.task.internal:Opening connections...
 DEBUG:teuthology.task.internal:connecting to ubuntu@plana06.front.sepia.ceph.com
 <hangs>

No clue what is going on here, but this fixes it!

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit f342baeb229b00914979cb923a7f2dab9197bc18)
Never use 'except:' without specifying an Exception.