Do not add DHCP info to subnet if DHCP is disabled
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

midonet/python-neutron-plugin-midonet,JoeMido/networking-midonet,midokura/python-neutron-plugin-midonet,yamt/networking-midonet,midonet/python-neutron-plugin-midonet,midokura/python-neutron-plugin-midonet,JoeMido/networking-midonet,yamt/networking-midonet