sentry.directory_listing failing on some clouds

[Stuart Bishop]

sentry.directory_listing has been failing for me, repeatably in the hpcloud test environment. Here is a traceback failing to list the root directory (ie. it is guaranteed to exist). This is after the environment has been successfully set up and several other tests successfully run on this unit.

DEBUG:runner:Jun 30 07:27:50 ======================================================================
DEBUG:runner:Jun 30 07:27:50 ERROR: test_external_mount (tests.test_integration.Test1UnitDeployment)
DEBUG:runner:Jun 30 07:27:50 ----------------------------------------------------------------------
DEBUG:runner:Jun 30 07:27:50 Traceback (most recent call last):
DEBUG:runner:Jun 30 07:27:50 File "/tmp/bundletester-b8Wd71/tmp3szD1A/tests/test_integration.py", line 296, in test_external_mount
DEBUG:runner:Jun 30 07:27:50 s.directory_contents('/')
DEBUG:runner:Jun 30 07:27:50 File "/tmp/bundletester-b8Wd71/tmp3szD1A/.venv3/lib/python3.4/site-packages/amulet/sentry.py", line 38, in directory_contents
DEBUG:runner:Jun 30 07:27:50 return self.directory_listing(*args)
DEBUG:runner:Jun 30 07:27:50 File "/tmp/bundletester-b8Wd71/tmp3szD1A/.venv3/lib/python3.4/site-packages/amulet/sentry.py", line 91, in directory_listing
DEBUG:runner:Jun 30 07:27:50 return self._run_unit_script("directory_listing.py {}".format(path))
DEBUG:runner:Jun 30 07:27:50 File "/tmp/bundletester-b8Wd71/tmp3szD1A/.venv3/lib/python3.4/site-packages/amulet/sentry.py", line 126, in _run_unit_script
DEBUG:runner:Jun 30 07:27:50 raise IOError(output)
DEBUG:runner:Jun 30 07:27:50 OSError: /tmp/juju-exec955499286/script.sh: line 1: /tmp/amulet/directory_listing.py: Not a directory
DEBUG:runner:Jun 30 07:27:50 ERROR subprocess encountered error code 126

I'm guessing ssh hostkey pollution, but have no actual evidence.