Pass $mysql_password through to gerrit class

The `$mysql_password` variable is used by the `secure.config.erb`
template in the gerrit class, but is not passed from
openstack_project::review -> openstack_project::gerrit -> gerrit.
Instead it uses dynamic scopeing to find the variable and won't work in
Puppet 3. This adds the full parameter passing for Puppet 3.

This commit also adds "Template uses" comments immediately preceding
resources declarations which use a `template()` function to describe all
variables used by the gerrit templates. This greatly helps with
debugging issues such as this.

Change-Id: I747e3e4623444c0345a7aed3732b7d316f1a7726
Merge "Pass $mysql_password through to gerrit class"