nuke: add missing import os

$ teuthology-nuke  -a . -r -u
Traceback (most recent call last):
  File "/home/ubuntu/bin/teuthology-nuke", line 9, in <module>
    load_entry_point('teuthology==0.0.1', 'console_scripts', 'teuthology-nuke')()
  File "/home/ubuntu/teuthology/teuthology/nuke.py", line 343, in main
    ifn = os.path.join(ctx.archive, 'info.yaml')
UnboundLocalError: local variable 'os' referenced before assignment

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #110 from ceph/wip-nuke-import-os

nuke: add missing import os