[LIBCLOUD-554] Add support for providing filters and VPC IDs to EC2: ex_list_networks()

EC2 API allows VPC ID and filters to be provided to list only specific VPCs. 
http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeVpcs.html
Add support for specifying filters and vpc ids when listing networks (vpc's) with ec2 driver's method ex_list_networks()