PBR will fail if any installed package has a bad distutils.command entry point

Steps to reproduce:

1) Create a new virtualenv.
2) Install pbr.
3) Install mock with --no-binary
mock needs pbr to install, the --no-binary is to force pip to use pbr rather than installing a cached wheel.
This should work fine.
4) Uninstall mock.
5) Install obspy
obspy has a bad distutils.command entry point, and cannot build its man pages. But it installs OK.
6) Install mock with --no-binary
This will likely fail. mock needs pbr to install, but pbr no longer works because of the bad entry point registered by obspy.

Stacktrace:
Collecting mock
  Using cached mock-2.0.0.tar.gz
    Complete output from command python setup.py egg_info:
    ERROR:root:Error parsing
    Traceback (most recent call last):
      File "/build/mhoyles/nmp_python_rpmbuild/TEST_ENV/lib/python2.7/site-packages/pbr/core.py", line 111, in pbr
        attrs = util.cfg_to_args(path, dist.script_args)
      File "/build/mhoyles/nmp_python_rpmbuild/TEST_ENV/lib/python2.7/site-packages/pbr/util.py", line 264, in cfg_to_args
        wrap_commands(kwargs)
      File "/build/mhoyles/nmp_python_rpmbuild/TEST_ENV/lib/python2.7/site-packages/pbr/util.py", line 566, in wrap_commands
        cmdclass = ep.resolve()
      File "/build/mhoyles/nmp_python_rpmbuild/TEST_ENV/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2264, in resolve
        module = __import__(self.module_name, fromlist=['__name__'], level=0)
    ImportError: No module named Help2Man
    error in setup command: Error parsing /tmp/pip-build-oeMFsg/mock/setup.cfg: ImportError: No module named Help2Man