Make rosetta use php-fpm via mod_proxy_fcgi

When Rosetta was running Ubuntu 16.04, it used mod_fastcgi for PHP. When it was upgraded to 18.04, mod_fastcgi wasn't available anymore. The Apache configuration is wrapped in <IfModule mod_fastcgi> so it was silently ignored; Apache didn't complain, but PHP didn't work either. It appears someone fixed that by installing mod_php, leaving the fastcgi config present but unused.

We shouldn't use mod_php in any of our servers. This commit removes mod_php and changes the Apache configuration to use mod_proxy_fcgi to proxy to php-fpm, like the other newer servers do.

I also changed the socket name in /etc/php/5.6/fpm/pool.d/capacity.conf from php5.5-fpm-capacity.sock to php5.6-capacity-fpm.sock, to have the proper version number and be consistent with other servers. That file is not yet in ansible so the change is not in this commit; I already changed it manually on rosetta.