Depracted warnings

[hbokh]

Multiple deprecated warnings for "$db_*"-entries in the files wp-config.erb and create_wordpress_db.erb.
Example:

warning: Dynamic lookup of $db_name ... wp-config.erb:19 is deprecated. For more information, see http://docs.puppetlabs.com/guides/scope_and_puppet.html. To see the change in behavior,
use the --debug flag.

[jonhadfield]

Changed templates to use instance variables as recommended: http://docs.puppetlabs.com/guides/templating.html.