Do not add DHCP info to subnet if DHCP is disabled

Currently the midonet plugin will add default DHCP information to
each subnet even if the subnet is configured to have DHCP disabled.
This change addresses this by checking the DHCP settings while
creating a subnet, and only adding DHCP information if DHCP is
enabled.

Closes-bug #1230073

Change-Id: I2c422866c60f505df30bfa18bfe8d03599665b7a
Add test_handle_router_snat_rules_add_rules

Add a unit test to verify that snat rules are added and added in order.
So that it could be refactored without fear next time.

Closes-Bug: #1222660
Change-Id: I07e820cc28c9a6c139a8eed0917aef2cfe62638a
Merge "Add test_handle_router_snat_rules_add_rules"
Merge "Do not add DHCP info to subnet if DHCP is disabled"