This was a flawed test. Previously the test passed in a fixture that
corresponded to a CLI config file, not an actual policy. The test
_should_ have been failing, but it wasn't. This commit adds a new
fixture.
Functionality is split into ExternalTokenHelper, which is used if a path
is given in a configuration file, and InternalTokenHelper which is used
otherwise. The internal helper no longer shells out to the same Vault
binary, instead performing the same actions with internal code. This
avoids problems using dev mode when there are spaces in paths or when
the binary is built in a container without a shell.
Fixes#850 among others
