imp] res.config.installer: set already installed modules to readonly, as res.config.installer can't uninstall them.
imp] res.config.installer: set already installed modules to readonly, as res.config.installer can't uninstall them.

Also, remove already installed modules from the set of modules to
install, and add a few docstrings to clarify the purpose of
overloading default_get and fields_get, and how _modules_to_install
works

xrg/openerp-server,splbio/openobject-server,vnc-biz/openerp-server,gisce/openobject-server,xrg/openerp-server,ovnicraft/openerp-server,splbio/openobject-server,MarkusTeufelberger/openobject-server,vnc-biz/openerp-server,ovnicraft/openerp-server,MarkusTeufelberger/openobject-server,gisce/openobject-server,gisce/openobject-server,splbio/openobject-server,MarkusTeufelberger/openobject-server