Description of problem:
Network creation while the specified default gateway is out of the network's range is possible and pass successfully

Version-Release number of selected component (if applicable):
Folsom on RHEL6.4, FlatDHCPManager, openstack-nova-2012.2.2-6.el6ost.noarch

Steps to Reproduce:
1. Create a network where the default gateway is out of the network's range, for instance:

$ nova-manage network create --label=net08_34 --fixed_range_v4=192.168.6.0/24 --bridge=br100 --project_id=a75203c3c0234ea0b3c16565e52af01e --num_networks=1 --gateway=192.168.7.7

Actual results:
Command passed successfully.

# mysql -u root nova -e "select id,cidr,netmask,bridge,gateway,broadcast,dhcp_start,label,bridge_interface from networks where label like 'net08_34';"
+----+----------------+---------------+--------+-------------+---------------+-------------+----------+------------------+
| id | cidr           | netmask       | bridge | gateway     | broadcast     | dhcp_start  | label    | bridge_interface |
+----+----------------+---------------+--------+-------------+---------------+-------------+----------+------------------+
|  8 | 192.168.6.0/24 | 255.255.255.0 | br100  | 192.168.7.7 | 192.168.6.255 | 192.168.6.2 | net08_34 | eth2             |
+----+----------------+---------------+--------+-------------+---------------+-------------+----------+------------------+


Expected results:
The command should fail with an exception which explains that the default gateway's address should be from within the network's range