Add time_to_live parameter into the barclamp

Add the parameter time_to_live (ceilometer.conf) into the barclamp,
so the user can decide when the meter lines are removed from the
database.

This patch was tested with a mongodb configuration.

This patch was made by Jiri (jsuchome). I only tested it and added
the parameter in the ceilometer.conf.erb template.
This is true but:
 * If the value is not set, ceilometer we consider -1 (default)
 * This is in chef/data_bags/crowbar/bc-template-ceilometer.json

    ```json
    "database": {
        "time_to_live": -1
     }
     ```

So there is some sense of default value here.