Issue #1567232 by Jose Reyero: API improvements for Variable Realm. Simplified realm hooks, now everything is in hook_variable_realm_info

Removed unneeded functions in variable_realm_union. Moved hook_variable_realm_info_alter() and related functions to .variable.inc file.

Issue #1567232 by Jose Reyero: API improvements for Variable Realm. Added some more options, auto generating union keys and variable list.

Per domain settings forms completely auto-generated, added needed information to hook_variable_realm_info().