Fix static route rhel

Hello,
Here is a dirty patch allowing us to specify static routes on Scientific Linux 5 (rhel 5-like).
It should address problems mentionned in #58.
It allows the following to be specified:
```
network_route { '10.8.0.0/24':
  ensure    => 'present',
  gateway   => '172.20.8.1',
  interface => 'eth1',
  network   => '10.8.0.0',
  netmask   => '255.255.255.0',
}
network_route { '10.8.1.0/24':
  ensure    => 'present',
  gateway   => '172.20.8.1',
  interface => 'eth1',
  network   => '10.8.1.0',
  netmask   => '24',
}
```
Genreating the following configuration in /etc/sysconfig/network-scripts/route-eth1:
```
10.8.0.0/24 via 172.20.8.1 dev eth1
10.8.1.0/24 via 172.20.8.1 dev eth1
```
So netmask can be provided as 255.255.255.0 or 24 (without useless resource refresh).