Manager now uses a late binding proxy for group plugin

[chungers]

Currently the manager functions as a stateful proxy for a stateless group plugin. It implements the group plugin SPI and intercepts user's api call to persist configs before dispatching the call out to the actual stateless group plugin for doing real work. As a result the current implementation expects a stateless group plugin to be running so that the manager can connect to it on startup.

This creates a problem of introducing ordering in how we start up the controllers. This PR fixes this by making a simple late-binding proxy for the group plugin. This way, plugin lookup / resolution doesn't take place on startup of the manager, but on the first call of the SPI methods.

This PR also sets the stage for implementing plugin activation by the manager, since the manager will be the first and only controller running and is responsible for starting up other plugins on-demand.

Signed-off-by: David Chung david.chung@docker.com

[codecov-io]

Current coverage is 64.77% (diff: 50.74%)

Merging #354 into master will decrease coverage by 0.15%

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