Logging api

Refactorings to support testing logging interface. @bernerdschaefer @peterbourgon 
per [the API](https://github.com/soundcloud/harpoon/blob/88504564ba1954296b5cdbfca502282607c4cd3e/doc/agent-api-v0.md#post-containersidaction), Restart and Stop don't take duration parameters (anymore). Maybe that's outside the scope of this PR?