Docker Support

Steps to reproduce:
0) $ pulp-admin docker repo create --repo-id busybox
1) $ pulp-admin docker repo delete --repo-id busybox

Actual results:

$ pulp-admin docker repo delete --repo-id busybox
This command may be exited via ctrl+c without affecting the request.

[\]
Running...

Task Failed

Pulp exception occurred: PulpExecutionException

coercing to Unicode: need string or buffer, NoneType found

coercing to Unicode: need string or buffer, NoneType found

In the log, this exception appears:

Error received removing distributor [docker_web_distributor_name_cli] from repo [busybox]
Traceback (most recent call last):
  File "/home/vagrant/devel/pulp/server/pulp/server/controllers/repository.py", line 311, in delete
    distributor_manager.remove_distributor(repo_id, repo_distributor['id'])
  File "/home/vagrant/devel/pulp/server/pulp/server/managers/repo/distributor.py", line 228, in remove_distributor
    distributor_instance.distributor_removed(transfer_repo, call_config)
  File "/home/vagrant/devel/pulp_docker/plugins/pulp_docker/plugins/distributors/distributor_web.py", line 162, in distributor_removed
    shutil.rmtree(repo_dir, ignore_errors=True)
  File "/usr/lib64/python2.7/shutil.py", line 228, in rmtree
    if os.path.islink(path):
  File "/usr/lib64/python2.7/posixpath.py", line 135, in islink
    st = os.lstat(path)
TypeError: coercing to Unicode: need string or buffer, NoneType found

Expected results:
The repo should be deleted without error.

Additonal information:
Similar errors have been reported against other plugins, and are linked in related issues.