More refactoring for role::mediawiki

* Rename lvs_pool parameter to pool and move it from role::mediawiki::common to
  role::mediawiki::webserver.
* Roles that include webserver don't need to include common as well, since
  webserver includes common.

Change-Id: Ibf47bebc80ac92412a8b59d8f331b8976d3c37ca