lpbuildbot sometimes needs to clean out a symlink before switching between devel and db-devel (public branch)

[Gary Poster]

Sometimes in the "Prepare base container" step of buildbot, just after running a devel test and switching to db-devel, we see the following error:

ln -sf /var/lib/buildbot/slaves/slave/lucid-db-devel/build /var/lib/buildbot/lp
 in dir /var/lib/buildbot/slaves/slave/lucid-db-devel/build (timeout 1200 secs)
 watching logfiles {}
 argv: ['ln', '-sf', '/var/lib/buildbot/slaves/slave/lucid-db-devel/build', '/var/lib/buildbot/lp']
 environment:
  APT_LISTCHANGES_FRONTEND=none
  CHARM_DIR=/var/lib/juju/units/buildbot-slave-0/charm
  DEBIAN_FRONTEND=noninteractive
  HOME=/var/lib/buildbot
  JUJU_AGENT_SOCKET=/var/lib/juju/units/buildbot-slave-0/.juju.hookcli.sock
  JUJU_CLIENT_ID=constant
  JUJU_PYTHONPATH=:/usr/lib/python2.7/dist-packages:/usr/lib/python2.7:/usr/lib/python2.7/plat-linux2:/usr/lib/python2.7/lib-tk:/usr/lib/python2.7/lib-old:/usr/lib/python2.7/lib-dynload:/usr/local/lib/python2.7/dist-packages:/usr/lib/pymodules/python2.7
  JUJU_RELATION=buildbot
  JUJU_RELATION_ID=buildbot:0
  JUJU_REMOTE_UNIT=buildbot-master/0
  JUJU_UNIT_NAME=buildbot-slave/0
  PATH=/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/sbin:/sbin:/bin
  PWD=/var/lib/buildbot/slaves/slave/lucid-db-devel/build
 closing stdin
 using PTY: False
ln: `/var/lib/buildbot/lp/build': cannot overwrite directory
program finished with exit code 1
elapsedTime=0.011064

I think we just need to rm -rf /var/lib/buildbot/lp before we run this step.