awsutils: use boto.connect_vpc for EC2 connection
awsutils: add get_subnet and get_subnets to EasyEC2
cluster: remove vpc_id from cluster settings

Only the subnet_id is required for VPC - the vpc_id needed when creating
a VPC security group can be fetched from the subnet object.

* Added Cluster.subnet property to simplify fetching/validation
* Moved VPC validation from Cluster.__init__ to ClusterValidator
* Updated VPC validation to check if the subnet exists
awsutils: add network_interfaces kwarg
awsutils: add network interface convenience methods

Added the following functions to EasyEC2:

* get_network_spec - proxy to boto's NetworkInterfaceSpecification

* get_network_collection - proxy to boto's NetworkInterfaceCollection
listclusters: show vpc and subnet ids
awsutils: use spinner when waiting for group propagation
cluster: validate subnet zone against cluster zone
cluster: validate the no. of available subnet ips
start: add --subnet/-N option (VPC)
cluster: add public_ips setting (default: True)

Sets whether or not to automatically assign a public ip address to all
nodes in a VPC cluster (defaults to True).

* Added --no-public-ips option to the start command. Public ips are
  enabled by default for VPC clusters.
* Moved all VPC configuration to a network interface spec. This enabled
  the new public_ips setting via associate_public_ip_address
* Updated spot request filters for VPC to use network-interface.*
  filters
* Added simple validation for public_ips setting
docs: add VPC section to configuration docs

* Added public_ips to list of cluster settings
* Added detailed section on using VPC with StarCluster
Merge branch 'vpc-refactor' into develop