queue: only bootstrap new checkouts

Until we figure out why bootstrap is getting stuck like this:

 9851 pts/7    S      0:03 /home/teuthworker/teuthology-master/virtualenv/bin/python ./teuthology-master/virtualenv/bin/teuthology-worker -v --archive-dir /var/lib/teuthworker/archive --tube plana --log-dir /var/lib/teuthworker/archive/worker_logs
 2075 pts/7    Z      0:00  \_ [git] <defunct>
 2112 pts/7    Z      0:00  \_ [git] <defunct>
 2138 pts/7    Z      0:00  \_ [bootstrap] <defunct>
 9852 pts/7    S      0:03 /home/teuthworker/teuthology-master/virtualenv/bin/python ./teuthology-master/virtualenv/bin/teuthology-worker -v --archive-dir /var/lib/teuthworker/archive --tube plana --log-dir /var/lib/teuthworker/archive/worker_logs
 2153 pts/7    Z      0:00  \_ [git] <defunct>
 2177 pts/7    Z      0:00  \_ [git] <defunct>
 2264 pts/7    Z      0:00  \_ [bootstrap] <defunct>
 9853 pts/7    S      0:03 /home/teuthworker/teuthology-master/virtualenv/bin/python ./teuthology-master/virtualenv/bin/teuthology-worker -v --archive-dir /var/lib/teuthworker/archive --tube plana --log-dir /var/lib/teuthworker/archive/worker_logs
 2141 pts/7    Z      0:00  \_ [git] <defunct>
 2276 pts/7    Z      0:00  \_ [git] <defunct>
 2305 pts/7    Z      0:00  \_ [bootstrap] <defunct>
 9854 pts/7    S      0:03 /home/teuthworker/teuthology-master/virtualenv/bin/python ./teuthology-master/virtualenv/bin/teuthology-worker -v --archive-dir /var/lib/teuthworker/archive --tube plana --log-dir /var/lib/teuthworker/archive/worker_logs
 7448 pts/7    Z      0:00  \_ [git] <defunct>
 7449 pts/7    Z      0:00  \_ [git] <defunct>
 7450 pts/7    Z      0:00  \_ [bootstrap] <defunct>
 7452 pts/7    Z      0:00  \_ [teuthology-resu] <defunct>
 9855 pts/7    S      0:01 /home/teuthworker/teuthology-master/virtualenv/bin/python ./teuthology-master/virtualenv/bin/teuthology-worker -v --archive-dir /var/lib/teuthworker/archive --tube plana --log-dir /var/lib/teuthworker/archive/worker_logs
 7712 pts/7    Z      0:00  \_ [git] <defunct>
 7713 pts/7    Z      0:00  \_ [git] <defunct>
 7714 pts/7    Z      0:00  \_ [bootstrap] <defunct>
 7716 pts/7    Z      0:00  \_ [teuthology-resu] <defunct>
 9856 pts/7    S      0:03 /home/teuthworker/teuthology-master/virtualenv/bin/python ./teuthology-master/virtualenv/bin/teuthology-worker -v --archive-dir /var/lib/teuthworker/archive --tube plana --log-dir /var/lib/teuthworker/archive/worker_logs
 2316 pts/7    Z      0:00  \_ [bootstrap] <defunct>
 9857 pts/7    S      0:03 /home/teuthworker/teuthology-master/virtualenv/bin/python ./teuthology-master/virtualenv/bin/teuthology-worker -v --archive-dir /var/lib/teuthworker/archive --tube plana --log-dir /var/lib/teuthworker/archive/worker_logs
 2340 pts/7    Z      0:00  \_ [bootstrap] <defunct>
 9858 pts/7    S      0:01 /home/teuthworker/teuthology-master/virtualenv/bin/python ./teuthology-master/virtualenv/bin/teuthology-worker -v --archive-dir /var/lib/teuthworker/archive --tube plana --log-dir /var/lib/teuthworker/archive/worker_logs
23188 pts/7    Z      0:00  \_ [bootstrap] <defunct>
 9859 pts/7    S      0:03 /home/teuthworker/teuthology-master/virtualenv/bin/python ./teuthology-master/virtualenv/bin/teuthology-worker -v --archive-dir /var/lib/teuthworker/archive --tube plana --log-dir /var/lib/teuthworker/archive/worker_logs

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