[IMP] res.config.installer: method returning just the names of the already-installed modules in the current installer.
[IMP] res.config.installer: method returning just the names of the already-installed modules in the current installer.

Method is public and RPC-accessible, use it in the existing methods using previous _already_installed but only needing the module names

akretion/openerp-server,akretion/openerp-server,akretion/openerp-server