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
createNetworkOffering and deleteNetworkOffering are only-available to ROOT-admins. I might add this at a later stage. Especially createNetworkOffering could be tricky based on the different network providers. I'd rather put some focus on CRUD VPC's.