Add missing parameter in call to _load_one_plugin

Stevedore 0.14 introduced a new paramter to the private method
ExtensionManager._load_one_plugin(). This broke our unit test suite, and
this patch adds that parameter to the call.

This patch supports both Stevedore < 0.14 and >= 0.14. Once we can pin
the version to >= 0.14, we should remove support for the old-style
call to _load_one_plugin.

For reference, we are directly calling this private method to provide a
mock-able extension manager and a corresponding fake extension for
testing the Conductor and the various extensions it loads.

Change-Id: Ib51f841af00548c3e20128f44a4b7a48422c1aa9
Closes-bug: #1273472