Glance: Enable passthrough configuration

This change enables config passthrough support for the following
Glance config files:

* glance-api.conf
* glance-registry.conf
* glance-cache.conf
* glance-scrubber.conf

Glance passthrough parameters can be injected into devtest by setting the
environment variable OVERCLOUD_EXTRA_CONFIG to a JSON structure such as:

{
    "glance-api": {
        "config": [
            {
                "section": "default",
                "values": [
                    {
                        "option": "qpid_port",
                        "value": "5672"
                    }
                ]
            }
        ]
    },
    "glance-registry": {
        "config": [
            {
                "section": "default",
                "values": [
                    {
                        "option": "limit_param_default",
                        "value": "100"
                    }
                ]
            }
        ]
    }
}

Change-Id: If0269b1935a10887ee3262e056d67a407131793a
blueprint: passthrough-config
Merge "Glance: Enable passthrough configuration"