Content Pack - Parameter Usability

## Proposed Functionality
The ability to integrate parameter values into fields with additional content.
This would allow for content packs to be created more dynamically and assist in automation of otherwise repetitive tasks.

## Current Behaviour
Currently, when you are utilising parameters within content packs, the value of the parameter is set as the field value and you unable to have additional content.

## Examples
A content pack which creates several streams inside the system, utilising a parameter value could allow for all the created streams to be prefixed or suffixed with a provided value.
In the UI, when assigning the parameter to the title config key, it would be good to be able to provide a string that references the parameter name which would then be swapped out for the value of that parameter during installation of the content pack.

## Context
Currently in order to achieve something similar to the above, you would have to install the content pack and then edit each stream individually in order to set the desired title.

## Environment
* Graylog Version: 3.0.1
* Elasticsearch Version: 6.7.0
* MongoDB Version: 4.0.9
* Operating System: Ubuntu 18.04