local provider tests fail with an empty home directory

[Clint Byrum]

The following failure happens when building juju in a clean chroot:

===============================================================================
[ERROR]
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1018, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 350, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/«BUILDDIR»/juju-0.5+bzr375/juju/providers/local/tests/test_provider.py", line 86, in test_bootstrap
    yield self.provider.bootstrap()
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1020, in _inlineCallbacks
    result = g.send(result)
  File "/«BUILDDIR»/juju-0.5+bzr375/juju/providers/local/__init__.py", line 122, in bootstrap
    raise ProviderError(str(e))
juju.errors.ProviderError: SSH authorized/public key not found.

juju.providers.local.tests.test_provider.LocalProviderTest.test_bootstrap
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1018, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 350, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/«BUILDDIR»/juju-0.5+bzr375/juju/providers/local/tests/test_provider.py", line 158, in test_connect
    yield self.provider.bootstrap()
  File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1020, in _inlineCallbacks
    result = g.send(result)
  File "/«BUILDDIR»/juju-0.5+bzr375/juju/providers/local/__init__.py", line 122, in bootstrap
    raise ProviderError(str(e))
juju.errors.ProviderError: SSH authorized/public key not found.

juju.providers.local.tests.test_provider.LocalProviderTest.test_connect
-------------------------------------------------------------------------------
Ran 1515 tests in 408.658s

FAILED (skips=7, errors=2, successes=1506)