Add static routes configuration (RFC3442)

[domcleal]

1. options should be defined in the main server configuration file
2. dhcp::pool should take an array of static routes that are written into each pool using above options

Based on https://github.com/theforeman/puppet-dhcp/commit/c22f4645c15d5a8ad1844346c514fa23adbee3a7.

[dvonessen]

I would also go with this option!