Solum/ConfigurableHandlerEndpoints

Configurable Handlers for Solum's RPC services
At present, Solum has three RPC services: worker, deployer, and conductor.

The three of them have similar architecture, including a handlers directory that contains a single endpoint each.

Since there's only a single handler per service, for now they're hardcoded. In the future, we will likely have alternative ways to build and deploy apps with Solum, and so operators should like to be able to configure which of the available options they would like to use.

To that end, I propose for each service, an additional "handler" config option, which simply names one of the available files in the respective "handlers" directory, and defaults to "default". For simplicity, each "default" handler could simply be a do-nothing agent that only acknowledges that the relevant function was called by logging.

cfg.StrOpt('handler',              default='default',               help='The deployer handler endpoint.')