Update DHCP subnet according to new API

MidoNet API takes enabled flag for DHCP subnet.  Use that field
to implement the feature.

Signed-off-by: Ryu Ishimoto <ryu@midokura.com>
Merge pull request #97 from midokura/MN1476

Update DHCP subnet according to new API

Reviewed-by: Joe Mills <joe@midokura.com>