MySQL: Enable passthrough configuration

This adds an extra MySQL conf file in etc/mysql/passthru.cnf
which is loaded last and therefore overrides all other MySQL options

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

    {
        "mysql": {
            "config": [
    	    {
    	        "section": "mysqld",
    		"values": [
    		    {
    		        "option": "innodb_buffer_pool_size",
    			"value": "200M"
    		    },
                        {
                            "option": "skip_name_resolve"
                        }
    	        ]
                }
    	]
        }
    }

This Json structure can be injected into devtest by setting the environment
variable OVERCLOUD_EXTRA_CONFIG:

export OVERCLOUD_EXTRA_CONFIG="{\"mysql\": {\"config\": [{\"section\": \
            \"mysqld\",\"values\": [{\"option\": \"innodb_buffer_pool_size\", \"value\": \
            \"200M\"},{\"option\": \"skip_name_resolve\"}]}]}}"

Change-Id: Ia2b963cdfb7029f72ac4773509258f2863f36b4b
Merge "MySQL: Enable passthrough configuration"