Plugins global config overwrites task-specific config

[IRCody]

When starting snapd with this config:

{
    "control" :{
        "plugins" : {
            "collector" : {
                "all" : {
                        "password" :"bogus",
                        "test" : true,
                        "user" : "really bogus"
                }
            },
            "publisher" : {
                "all" : {
                    "file" : "/tmp/whatever"
                }
            }
        }
    }
}

With this task. To reproduce start snapd with the above config and load snap-collect-mock1, snap-processor-passthru, and snap-publisher-file.

I would expect the config values passed to my plugins from this task to be what is in the task manifest for the conflicting values (password, user, file). What actually appears to be happening is that the values from global config are getting passed through where they conflict. Worth mentioning here that there is no way for a user to see what the 'merged' value of the config is between global config and task-specific config is which is addressed in #846.