Migrate beta appserver configs to role::beta::appserver

* Doing this with Bryan's blessing; Bryan and I will update the configs of labs
  hosts.
* Make the pool param for the webserver role nonoptional, since the remaining
  sub-roles that apply it all specify a value.
* Dissolve role::mediawiki::php; integrate it into ::common

Change-Id: Id0b28012f6b846bfb9f57184deb5485caa102fd0