Allow ovirt to implement their own pg version check

This doesn't fix the following bug but allows them to implement their
own adapter that opts-out of the manageiq pg version check and instead
implement their own via a check_version, something like this:

```ruby
module ActiveRecord
  module ConnectionHandling # :nodoc:
    def ovirt_postgresql_connection(config)
      conn_params = config.symbolize_keys
      ...rest of the existing rails 5.1 pg adapter method until...
      ConnectionAdapters::OvirtPostgreSQLAdapter.new(nil, logger, conn_params, config)
    end
  end
  module ConnectionAdapters
    class OvirtPostgreSQLAdapter < PostgreSQLAdapter
      def check_version
        raise "AAAAA" if postgresql_version > XXX
      end
    end
  end
end
```

https://bugzilla.redhat.com/show_bug.cgi?id=1734770

config/initializers/postgres_required_versions.rb

@@ -1,7 +1,10 @@
 ActiveRecord::ConnectionAdapters::PostgreSQLAdapter.prepend Module.new {
   def initialize(*args)
     super
+    check_version if respond_to?(:check_version)
+  end
 
+  def check_version
     msg = "The version of PostgreSQL being connected to is incompatible with #{Vmdb::Appliance.PRODUCT_NAME} (10 required)"
 
     if postgresql_version < 90500