* use util.subp from inside parse_shell_config,
and adjust exception handling accordingly.
* add 'switch_user_cmd' as a callback function to pass
to parse_shell_config, which allows us to mock this to avoid
'sudo' when running test cases.
Basically the test cases just return '[]' here.
* fix some pylint
* handle empty 'content' in parse_shell_config and remove
the protection that was present.
7608bc2e65