Cloudstack Driver - Create/Delete Network functionality + tests

Functions added on the cloudstack driver:
 
ex_list_network_offering
ex_create_network
ex_delete_network

New Object:
CloudStackNetworkOffering

And some additions to the CloudStackNetwork dictionary
netmask
gatway
vpcid
For consistency with existing code and other drivers, please use `_` to separate words in the argument names - display_text, guest_ip_type, service_offerring_id, for_vpc, etc.