[LIBCLOUD-593] CloudStack driver's create_node fails to join network_id ...

CloudStack driver's create_node accept an argument "networks" it is used to specify networks the node will be connected to.
networks is a list of CloudStackNetwork instances and their id is joined to construct a argument for CloudStack API's deployVirtualMachine in create_node. It is OK when network id is string but some implementation uses integer network id.
When network id is integer, the code like following fails.
networks = driver.ex_list_networks()
node = driver.create_node(location, image, size, networks=networks)
This patch convert network id into str explicitly before joining network ids.

https://issues.apache.org/jira/browse/LIBCLOUD-593