Fix nova-network settings in multi_host mode

This patch is aim to fix two errors in nova-network setting:
 - When multi_host is enable, enable_network_service should be true.
Otherwise, it will not install nova-network in controller.
 - When multi_host is disable, related param in nova.conf should set
as false.

Fixes Bug #1212093

Change-Id: If62427b3ac7ee744c16a6f4e67803cf1e233f374
Merge "Fix nova-network settings in multi_host mode"