Add $status_path parameter to change mod_status url

This change lets the user decide the url to use when accessing the status, instead of hardcoding it to '/server-status', which is the default.

This is #887 with 1 commit, and changes in theREADME and in https://github.com/puppetlabs/puppetlabs-apache/blob/master/spec/classes/mod/status_spec.rb