Adds passthrough config for cinder rootwrap

This patch adds a passthrough.conf to the cinder rootwrap.d directory
which supports passthrough config.

This is useful for folks who want to add custom logging or other conf
to their rootwrap.

ExtraConfig snippet looks like this:
{
  "cinder": {
    "rootwrap_config": [
      {
        "section": "SECTIONNAME",
        "values": [
          {
            "option": "OPTIONNAME",
            "value": "VALUENAME"
          }
        ]
      }
    ]
  }
}
Change-Id: Ifbc49a266c95e7032ec33e2b3cd48b9f551a3efe
blueprint: passthrough-config
Merge "Adds passthrough config for cinder rootwrap"