this will initialize the plugin multisite. It will

- add a the "multisite_setting" default metadata value that will be used when adding a new site
- add a 'member_of_site' relationship to the site owning them to every registered member
- create a bunch of plugin objects that will be used for the plugin management
- take care of updating sites already runnuing with Beta 1.2 version

Default domain name : the default domain name that will be used for the subdomains creation
Network : the default network name or "all" or "no"