change write_config to write_include, write_config doesn't work on ubunt...

Looked like there's a pull request for this [1] a while ago but somehow it doesn't make it to the master. I've submitted my own change as it's blocking our deployment.


[1] https://github.com/spheromak/dhcp-cook/pull/4

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Error executing action `add` on resource 'dhcp_group[sf-mgmt]'
================================================================================


NameError
---------
Cannot find a resource for write_config on ubuntu version 12.04


Cookbook Trace:
---------------
/var/chef/cache/cookbooks/dhcp/providers/group.rb:40:in `block in class_from_file'