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
Instead of using kwargs, please explicitly declare `gateway`, `netmask` and `vpc_id` argument in the method signature.

Using kwargs in cases like this makes programmatic introspection and many other things hard / impossible - https://libcloud.readthedocs.org/en/latest/development.html#don-t-abuse-kwargs