puppetmaster-common: add recommends for stored configs

Recommend ruby-activerecord and ruby-activerecord-deprecated-finders, as
they are needed for the (optional) stored configs functionality. This is
needed because ruby-activerecord-deprecated-finders is not a
direct dependency and is not straightforward to figure out.

Note that users will also have to install the appropriate database
adapter (e.g. ruby-sqlite3, ruby-mysql) to make stored configs work.