Update params to be able to disable epel class optionally.

I would like to add an optional param to be able to disable the 'include epel.'
For my use I keep a local epel repo on site and have my own class to handle it. Others might find it useful to drop this requirement as well if they have an existing way to get the ejabberd package.

With the $include_epel defaulted to true the class will operate the same, unless you are an oddball like myself.