User should be able to rerun uft-flocker-config cluster.yml

[Stephenitis]

if error occurs during this operation user might encounter that command exiting with the following:

Error: Unable to write certificate file. File exists /pwd/cluster.crt
main function encountered error
Traceback (most recent call last):
  File "/opt/flocker/bin/flocker-config", line 9, in <module>
    load_entry_point('UnofficialFlockerTools==0.5', 'console_scripts', 'flocker-config')()
  File "/opt/flocker/lib/python2.7/site-packages/unofficial_flocker_tools/config.py", line 147, in _main
    react(main, sys.argv[1:])
  File "/opt/flocker/lib/python2.7/site-packages/twisted/internet/task.py", line 875, in react
    finished = main(_reactor, *argv)
  File "/opt/flocker/lib/python2.7/site-packages/twisted/internet/defer.py", line 1253, in unwindGenerator
    return _inlineCallbacks(None, gen, Deferred())
--- <exception caught here> ---
  File "/opt/flocker/lib/python2.7/site-packages/twisted/internet/defer.py", line 1107, in _inlineCallbacks
    result = g.send(result)
  File "/opt/flocker/lib/python2.7/site-packages/unofficial_flocker_tools/config.py", line 21, in main
    c.run("flocker-ca initialize %s" % (c.config["cluster_name"],))
  File "/opt/flocker/lib/python2.7/site-packages/unofficial_flocker_tools/utils.py", line 208, in run
    result = subprocess.check_output(command, shell=True)
  File "/usr/lib/python2.7/subprocess.py", line 573, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command 'flocker-ca initialize coreos-flocker' returned non-zero exit status 1

causes the user to have to manually ssh in and debug or start all over.