Introduce a helper to create and start the volume service.
Define that new interface.
Remove obsolete sys_module stuff from VolumeManagerScript.  And likewise for related tests.
Delete a lot more superfluous tests.  Fix an implementation bug introduced a little while ago which caused deployer to always use a real gear client.
There is no longer a _deployer.  But there is a _gear_client.

Implementation details woo hoo.
Ahem.
Deployer creates the default.  Just verify a default of None at this layer.
Put the VolumeService factory back, we still want to create these.
fix flakes
Add a test for the SystemExit behavior.
Add a test for the bytes written to stderr when the volume service cannot be created.
Add a test for a correctly constructed, running VolumeService returned from the construction method.
Add a test for `_service_factory` being respected and use it to add another test for `main` calling the other object's `main` method.
Test the default sys module.
Fix busted docstring.
Perhaps this makes the test happy when run as root.
Use a made up path with bad permissions.
Give test_details_written the same treatment.
Dedent "overindented" lines.
Some doc tweaks