nuke: s/run_name/name/

This matches an existing argument (with the same meaning) and
avoids an error like

2013-10-01T17:20:35.395 CRITICAL:root:  File "/var/lib/teuthworker/teuthology-master/virtualenv/bin/teuthology", line 9, in <module>
    load_entry_point('teuthology==0.0.1', 'console_scripts', 'teuthology')()
  File "/home/teuthworker/teuthology-master/teuthology/run.py", line 235, in main
    nuke(ctx, log, ctx.lock)
  File "/home/teuthworker/teuthology-master/teuthology/nuke.py", line 391, in nuke
    if ctx.run_name:

2013-10-01T17:20:35.395 CRITICAL:root:AttributeError: 'Namespace' object has no attribute 'run_name'

Signed-off-by: Sage Weil <sage@inktank.com>